Immobilized Selenium in a Porous Carbon With the Presence of Oxygen, a Method of Making, and Uses of Immobilized Selenium in a Rechargeable Battery

ABSTRACT

In a method of preparing an immobilized selenium system or body, a selenium-carbon-oxygen mixture is formed. The mixture is then heated to a temperature above the melting temperature of selenium and the heated mixture is then cooled to ambient or room temperature, thereby forming the immobilized selenium system or body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/US2020/017509 filed Feb. 10, 2020, which claims the benefit of U.S. Provisional Patent Application No. 62/802,929 filed Feb. 8, 2019. In addition, this application is a continuation-in-part of U.S. patent application Ser. No. 15/434,655 filed Feb. 16, 2017, which is a continuation-in-part of U.S. patent application Ser. No. 15/262,407 filed on Sep. 12, 2016, issued as U.S. Pat. No. 10,340,507 on Jul. 2, 2019. U.S. patent application Ser. No. 15/434,655 also claims the benefit of U.S. Provisional Patent Application No. 62/367,314 filed on Jul. 27, 2016, U.S. Provisional Patent Application No. 62/364,113 filed on Jul. 19, 2016, U.S. Provisional Patent Application No. 62/296,286 filed on Feb. 17, 2016, and Chinese Patent Application No. CN 201510608018.4 filed on Sep. 22, 2015. The disclosures of each of the foregoing applications are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present application relates to the field of lithium secondary batteries of high energy density. More particularly, the application relates to a method of preparing carbon-selenium nanocomposite materials and their applications. The present invention also relates to immobilized selenium comprising selenium and carbon. It also relates to a method of making and the utility of the immobilized selenium. One of the uses of the immobilized selenium is in a rechargeable battery. The present invention also relates to a rechargeable battery that can perform discharge-charge cycling at a fast rate (e.g., 10 C-rate) with a minimum level of capacity fading while being able to substantially recover its electrochemical performance, such as specific capacity, when charged at a low rate such as at 0.1 C-rate.

Description of Related Art

With the increasing human demand for energy, secondary batteries with high mass specific energy and high volumetric energy density, such as lithium-sulfur batteries and lithium-selenium batteries, have attracted widespread interests. Group 6A elements in the Periodical Table, such as sulfur and selenium, have shown two-electron reaction mechanisms in the electrochemical reaction process with lithium. Despite the theoretical mass specific capacity of selenium (675 mAh/g) being lower than that of sulfur (1675 mAh/g), selenium has a higher density (4.82 g/cm³) than sulfur (2.07 g/cm). Therefore the theoretical volumetric energy density of selenium (3253 mAh/cm³) is close to the theoretical volumetric energy density of sulfur (3467 mAh/cm³). At the same time, as compared with sulfur, which is close to an electrically insulating material, selenium is semi-conductive electrically and shows better electrical conductive properties. Therefore, as compared to sulfur, selenium can demonstrate a higher level of activity and better utilization efficiency even at a higher loading level, leading to high energy and power density battery systems. Moreover, a selenium-carbon composite can have a further improvement in the electrical conductivity over sulfur-carbon composite to obtain a higher activity electrode material.

As described in patent publication no. CN104393304A, by passing hydrogen selenide gas through a graphene dispersion solution, the solvent heat reduces the graphene oxide into graphene while oxidizes the hydrogen selenide into selenium. The thus prepared selenium graphene electrode material pairs with an ether electrolyte system, 1.5M lithium bi-trifluoromethane sulfonimide (LiTFSI)/1,3-dioxolane (DOL)+dimethyl ether (DME) (volume ratio 1:1); the charging specific capacity reaches 640 mAh/g (approaching selenium theoretical specific capacity) in the first cycle. But in the charge-discharge process. polyselenide ions dissolve in the electrolyte, showing significant amounts of the shuttle effect, which causes the subsequent capacity decay. At the same time. the procedures for preparing the graphene oxide raw material that is used in this process are complicated, not suitable for industrial production.

Patent CN104201389B discloses a lithium-selenium battery cathode material, utilizing a nitrogen-containing layered porous carbon composite current-collector which was compounded with selenium. In preparing the nitrogen-containing layered porous carbon composite current collector, nitrogen-containing conductive polymer is first deposited or grown on the surface of a piece of paper, followed by alkali activation and high temperature carbonization, resulting in a nitrogen-containing layered porous carbon composite current collector with carbon fiber as network structure that supports itself; and such nitrogen-containing layered porous carbon composite current collector is then further compounded with selenium. The deposition method for preparing a conductive polymer is complicated and the process for film formation or growth is hard to control. The preparation process is complicated, which associates with undesirably high costs.

Moreover, the demand for a long life, high-energy-density and high-power-density rechargeable battery with the ability of being charged and discharged at a fast rate is ever increasing in electronics, electric/hybrid vehicles, aerospace/drones, submarines, and other industrial, military, and consumer applications. Lithium ion batteries are examples of rechargeable batteries in the above-mentioned applications. However, the need for better performance and cycling capability have not been filled with lithium ion batteries as the technology in lithium ion battery has matured.

Atomic oxygen has an atomic weight 16 and has a capability for 2 electron transfers. A lithium-oxygen rechargeable battery has been studied for the purpose of making high energy density batteries. When a battery involves an oxygen cathode that pairs with lithium or sodium metal as an anode, it has the greatest stoichiometric energy density. However, a majority of technical problems in the Li/Na-Oxygen battery remains unresolved.

Elemental sulfur is also in the oxygen group and has the second highest energy density (after oxygen) when paired with a lithium or sodium metal anode. A lithium-sulfur or sodium-sulfur battery has been widely studied. However, polysulfide ions (intermediates) that form during the Li—S or Na—S battery discharging process dissolve in the electrolyte and shuttle from cathode to anode. Upon reaching the anode, polysulfide anions react with lithium or sodium metal, resulting in a loss of energy density, which is undesirable for a battery system. In addition, sulfur is an insulator, which requires a high loading level of carbon materials to achieve a minimum level of electrical conductivity. Due to the extremely low electrical conductivity of sulfur, a Li/Na—S rechargeable battery is very difficult to discharge or charge at a fast rate.

SUMMARY OF THE INVENTION

Disclosed herein is a process to prepare a two-dimensional carbon nanomaterial, which has a high degree of graphitization. The two-dimensional carbon nanomaterials are compounded with selenium to obtain a carbon-selenium composite material, which is used as a cathode material that pairs with anode material containing lithium, resulting in a lithium-selenium battery that has a high energy density and stable electrochemical performances. A similar process can be used to further assemble a pouch cell, which also demonstrates excellent electrochemical properties.

Also disclosed is a method to prepare selenium-carbon composite material with readily available raw materials and simple preparation procedures.

The selenium-carbon composite material described herein can be obtained from a preparation method that comprises the following steps:

(1) Carbonize alkali metal organic salts or alkaline earth metal organic salts in high temperature. and then wash with dilute hydrochloric acid or some other acids, and dry to obtain a two-dimensional carbon material;

(2) Mix the two-dimensional carbon material obtained in step (1) with selenium in organic solution, heat and evaporate the organic solvent, and then achieve compounding selenium with the two-dimensional carbon material through a multi-stage heat ramping and soaking procedure to obtain carbon-selenium composite.

In step (1), the alkali metal organic salt can be selected from one or several of potassium citrate, potassium gluconate, and sucrose acid sodium. The alkaline earth metal organic salt can be selected from one or both of calcium gluconate, and sucrose acid calcium. The high temperature carbonization can be performed at 600-1000° C. desirably, 700-900° C.; carbonation time for 1-10 hours, desirably for 3-5 hours.

In step (2), the organic solvent can be selected from one or several of ethanol, dimethylsulfoxide (DMSO), toluene. acetonitrile. N,N-dimethylformamide (DMF), carbon tetrachloride. and diethyl ether or ethyl acetate; multi-stage heat ramping and soaking section can be referred as to a ramping rate 2-10° C./min, desirably 5-8° C./min, to a temperature between 200 and 300° C. desirably between 220 and 280° C., followed by soaking at the temperature for 3-10 hours, desirably, 3-4 hours; then continue to heat up to 400-600° C., desirably, 430-460° C. followed by soaking for 10-30 hours, desirably 15-20 hours.

Also disclosed herein is a lithium-selenium secondary battery that comprises the carbon-selenium composite materials. The lithium-selenium secondary battery can further include: a lithium-containing anode, a separator, and an electrolyte.

The lithium-containing anode may be one or several of lithium metal, a lithiated graphite anode, and lithiated silicon carbon anode materials (through assembling the graphite and silicon-carbon anode materials and lithium anode into a half cell battery, discharge to prepare lithiated graphite anode and lithiated silicon-carbon anode materials). The separator (membrane) can be a commercially available membrane, such as, without limitation, a Celgard membrane, a Whatman membrane, a cellulose membrane, or a polymer membrane. The electrolyte can be one or several of a carbonate electrolyte, an ether electrolyte, and ionic liquids.

The carbonate electrolyte can be selected from one or several from diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and propylene carbonate (PC); and the solute can be selected from one or several from lithium hexafluoro phosphate (LiPF₆), lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). lithium perchlorate (LiClO₄) and lithium bis(fluorosulfonyl) imide (LiFSI).

For the ether electrolytic solution, the solvent can be selected one or several from 1,3-dioxolane (DOL), ethylene glycol dimethyl ether (DME) and triethylene glycol dimethyl ether (TEGDME); and the solute can be selected in one or more from lithium hexafluorophosphate (LiPF₆). lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI). lithium perchlorate (LiClO₄) and lithium bis-fluorosulfonylimide (LiFSI).

For ionic liquids. the ionic liquid can be one or more room temperature ionic liquid [EMIm] NTf2 (1-ethyl-3-methylimidazolium bis trifluoromethane sulfonimide salt), [Py13] NTf2 (N-Propyl-N-methylpyrrolidine bis trifluoromethane sulfonimide salt). [PP13] NTf2 (N-propyl-methylpiperidine alkoxy-N-Bis trifluoromethane sulfonimide salts); and the solute can be selected in one or more from lithium hexafluorophosphate (LiPF₆), bis(trifluoromethylsulfonyl) imide (LiTFSI). lithium perchlorate (LiClO₄) and lithium bis fluorosulfonylimide (LiFSI).

Also described herein is a lithium-selenium pouch cell battery that includes the carbon selenium composite material.

Compared to the prior art, with respect to the method for preparing selenium carbon composite material disclosed herein, the two-dimensional carbon material has the advantages that the raw materials are readily available at low cost, the preparation method is simple, highly practical and suitable for mass production. and the obtained selenium carbon composite material exhibits excellent electrochemical properties.

Also disclosed herein is immobilized selenium (an immobilized selenium body) comprising selenium and a carbon skeleton. The immobilized selenium comprises at least one of the following: (a) requires gaining enough energy for a selenium particle to reach a kinetic energy of ≥9.5 U/mole, ≥9.7 U/mole, ≥9.9 U/mole, ≥10.1 U/mole, ≥10.3 U/mole, or ≥10.5 U/mole to escape the immobilized selenium system; (b) a temperature of 490° C. or higher, ≥500° C., ≥510° C., ≥520° C., ≥530° C., ≥540° C., ≥550° C., or ≥560° C. is required for selenium particles to gain enough energy to escape the immobilized selenium system; (c) the carbon skeleton has a surface area (with pores less than 20 angstroms) ≥500 m²/g, ≥600 m²/g, 700 m²/g, ≥800 m²/g, ≥900 m²/g, or ≥1,000 m²/g; (d) the carbon skeleton has a surface area (for pores between 20 angstroms and 1000 angstroms) 20% or less, 15% or less, 10% or less, 5% or less, 3% or less, 2% or less, 1% or less of the total surface area.

Also disclosed herein is a cathode or a rechargeable battery comprising immobilized selenium. The selenium may be doped with other elements, such as, but not limited to, sulfur.

Also disclosed herein is a composite including selenium and carbon comprising platelet morphology with as aspect ratio of ≥1, ≥2, ≥5, ≥10, or ≥20.

Also disclosed herein is a cathode including a composite comprising selenium and carbon and comprising platelet morphology with an aspect ratio of ≥1, ≥2, ≥5, ≥10, or ≥20. Also disclosed herein is a rechargeable battery including a composite comprising selenium and carbon and comprising platelet morphology with the foregoing aspect ratio.

Also disclosed herein is a rechargeable battery comprising a cathode, an anode, a separator, and an electrolyte. The rechargeable battery can be charged at 0.1 C, 0.2 C, 0.5 C, 1 C, 1.5 C, 2 C, 3 C, 4 C, 5 C, 6 C, 7 C, 8 C, 9 C, 1° C. or faster. The cathode can comprise at least one element of a chalcogen group such as selenium, sulfur, tellurium, and oxygen. The anode can comprise at least one element of an alkali metal, alkali earth metals, and a group IIIA metal or metals. The separator can comprise an organic separator or an inorganic separator whose surface can optionally be modified. The electrolyte can comprise at least one element of alkali metals, alkali earth metals, and a group IIIA metal or metals. The solvent in the electrolyte solution can comprise organic solvents, carbonate-based, ether-based, or ester-based.

The rechargeable battery may have a specific capacity of 400 mAh/g or higher, 450 mAh/g or higher, 500 mAh/g or higher, 550 mAh/g or higher, or 600 mAh/g or higher. The rechargeable battery may be able to undergo electrochemical cycling for 50 cycle or more, 75 cycles or more, 100 cycles or more, 200 cycles or more, etc. The rechargeable battery may retain a battery specific capacity greater than 30%, greater than 40%, greater than 50%, greater than 60%, greater than 70%, or greater than 80% of the 2^(nd) discharge specific capacity at a cycling rate of 0.1 C after conducting high C-Rate charge-discharge cycling (e.g., 5 cycles at 0.1 C, 5 cycles at 0.2 C, 5 cycles at 0.5 C, 5 cycles at 1 C, 5 cycles at 2 C, 5 cycles at 5 C, and 5 cycles at 10 C). The rechargeable battery may have a Coulombic efficiency ≥50%, ≥60%, ≥70%, ≥80%, or ≥90%. The rechargeable battery can be used for electronics, an electric or hybrid vehicle, an industrial application, a military application such as a drone, an aerospace application, a marine application, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a 50,000× scanning electron microscope photograph of the carbon material of Example 1;

FIG. 2 is a 0.1 C charge and discharge curve of the lithium selenium battery of Example 1;

FIG. 3 is a 0.1 C charge and discharge curve of the lithium selenium battery of comparative Example 2;

FIG. 4 is an optical image of the pouch cell battery of Example 1;

FIG. 5 is a 0.05 C charge and discharge curve of the pouch cell battery of Example 1;

FIG. 6 is a flow diagram of a process of making immobilized selenium;

FIG. 7 is a scanning electron microscope image of a carbon skeleton prepared by the process of Example 9;

FIG. 8 is X-ray diffraction patterns for the carbon skeleton prepared by the process of Example 9;

FIG. 9 is Raman spectrum of the carbon skeleton that was prepared by the process of Example 9;

FIG. 10A is a graph of cumulative and incremental surface area for the carbon skeleton prepared by the process of Example 9;

FIG. 10B is a graph of cumulative and incremental pour volumes for the carbon skeleton prepared by the process of Example 9;

FIG. 1A is a graph of TGA analysis for the immobilized selenium that was prepared by the process of Example 10;

FIG. 11B is a graph of TGA analysis for non-immobilized selenium samples prepared by the process of Example 10 using Se-Super P carbon and Se-graphite;

FIG. 11C is a graph of TGA analysis of the non-immobilized selenium sample prepared using the Se-Super P carbon (FIG. 11B) under a flow of argon gas and at heating rates of 16° C./min and 10° C./min;

FIG. 11D is a graph of rate constants for non-immobilized selenium (Se-Super P composite-solid line), and 2 different samples of immobilized selenium (228-110 (dotted line) and 115-82-2 (dashed line)) prepared by the process of Example 10;

FIG. 12 is a graph of Raman spectrum of the immobilized selenium prepared by the process of Example 10;

FIG. 13 is a graph of X-ray diffraction patterns for the immobilized selenium prepared by the process of Example 10;

FIG. 14 is an SEM image of the immobilized selenium prepared by the process of Example 10;

FIG. 15 is an exploded view of a coin cell battery including a cathode prepared according to the process of Example 11 or Example 13;

FIG. 16 are graphs of cycling test results for a first lithium-selenium coin cell battery (0.1 C) (FIG. 16A—left) and a second lithium-selenium coin cell battery (0.1 C and then 1 C) (FIG. 16A—right) of the type shown in FIG. 15 that were prepared by the process of Example 12;

FIG. 17 are graphs of cycling tests for a lithium-selenium coin cell battery of the type shown in FIG. 15 that was prepared by the process of Example 12 at different cycling rates;

FIG. 18 are graphs of 0.1 C cycling test results for a lithium-sulfur-doped-selenium coin cell battery of the type shown in FIG. 15 made in accordance with Example 13 with a polymer separator;

FIG. 19 are graphs of 1 C cycling test results for a lithium-sulfur-doped-selenium coin cell battery of the type shown in FIG. 15 made in accordance with Example 13 with a polymer separator.

FIG. 20 is SEM images of a three-dimensional interconnected thin wall porous carbon nanomaterial produced from glucose;

FIG. 21 is BET N₂ isotherms (a) and Pore Size Distributions (c) for three-dimensional interconnected thin wall porous carbon nanomaterials produced from glucose;

FIG. 22 are SEM images of a three-dimensional interconnected thin wall porous carbon nanomaterial produced from soybean mills; and

FIG. 23 are BET N₂ isotherms (b) and Pore Size Distributions (d) for three-dimensional interconnected thin wall porous carbon nanomaterials produced from soybean mills.

DESCRIPTION OF THE INVENTION

In conjunction with the specific examples, the present invention will be further described below. Unless otherwise specified, the experimental methods in the following examples are all conventional; the reagents and materials are all available from commercial sources.

Example 1

(A) Preparation of Selenium Carbon Composite Material

After grinding and milling, an appropriate amount of potassium citrate is calcined at 800° C. for 5 hours under an inert atmosphere. and cooled to room temperature. Washed with dilute hydrochloric acid to a neutral pH; filtered and dried to give a two-dimensional carbon nanomaterial (FIG. 1); according to the mass ratio of 50:50. weigh the two dimensional carbon material and selenium, and then stir and mix with the ethanol solution of selenium uniformly; after solvent evaporation, dry the mixture in dry oven; the dried mixture was heated at 5° C./min to 240° C. and soaked for 3 hours; then continues to heat up at 5° C./min to 450° C.; soaked for 20 hours; cooled to room temperature, which resulted in the selenium carbon composite material.

(B) Preparation of the Selenium Carbon Composite Cathode

The above-prepared selenium carbon composites are mixed with carbon black Super P (TIMCAL) and binder CMC/SBR (weight ratio 1:1) along with water by a fixed formulation by pulping, coating. drying and other procedures to obtain a selenium carbon composite cathode.

(C) Assembling Lithium—Selenium Battery

The above-prepared selenium carbon composite cathode. lithium foil as anode. Celgard diaphragm as separator and 1M LiPF₆ in EC/DMC as the electrolyte were assembled into a lithium selenium coin cell battery and a lithium selenium pouch cell battery (FIG. 4).

(D) Lithium-Selenium Battery Test

Use a charge-discharge apparatus to perform a constant current charge-discharge test on the lithium-selenium coin cell battery and lithium selenium pouch cell battery. Test voltage range is between 1.0 and 3.0 V and test temperature is 25° C. Discharge specific capacity and the level of charge-discharge current are standardly calculated based on the mass of selenium. The charge-discharge current is 0.1 C or 0.05 C. The lithium selenium coin charge and discharge curve is shown in FIG. 2. the specific test results are shown in Table 1 below. Lithium selenium pouch cell test results are shown in FIG. 5.

Example 2

Conditions are the same as in Example 1, with the exception that the raw material carbonized for two-dimensional carbon is sodium citrate. Battery test results are summarized in Table 1 below.

Example 3

Conditions are the same as in Example 1, with the exception that the raw material carbonized for two-dimensional carbon is potassium gluconate. Battery test results are summarized in Table 1 below.

Example 4

Conditions are the same as in Example 1, with the exception that the high-temperature carbonization temperature for the carbon material is 650° C. Battery test results are summarized in Table 1 below.

Example 5

Conditions are the same as in Example 1, with the exception that the dried mixture was heated at 5° C./min to 300° C. and soaked at this temperature for 3 hours. Battery test results are summarized in Table 1 below.

Example 6

Conditions are the same as in Example 1, with the exception that the dried mixture was heated at 5° C./min to 240° C. and soaked at this temperature for 3 hours. then continued to heat up to 600° C. and soaked at this constant temperature for 20 hours. Battery test results are summarized in Table 1 below.

Example 7

Conditions are the same as in Example 1, with the exception that the lithium-Se battery is packed with lithiated graphite anode, instead of the lithium anode sheet. Battery test results are summarized in Table 1 below.

Example 8

Conditions are the same as in Example 1, with the exception that the lithium-Se battery is packed with lithiated silicon carbon anode, instead of the lithium anode sheet. Battery test results are summarized in Table 1 below.

Comparative Example 1

Conditions are the same as in Example 1, with the exception that the use of polyacrylonitrile as the raw material. Battery test results are summarized in Table 1 below.

Comparative Example 2

Conditions are the same as in Example 1; with the exception that a one-step compound method is used to prepare the selenium and carbon composite. In this example, the dried selenium carbon mixture was heated at 5° C./min to 500° C. and soaked at this temperature for 23 hours to obtain selenium carbon composite material. The charge-discharge curve of a battery made from the thus obtained selenium carbon composite material is shown in FIG. 3; the battery test results are summarized in Table 1 below.

TABLE 1 summarized Battery Test Results The first cycle The first cycle charge capacity/ discharge the first cycle capacity discharge capacity The 50^(th) cycle Numbering (mAh/g) (%) capacity (mAh/g) Example 1 1,050 78.1 756 Example 2 940 74.6 672 Example 3 962 75.3 683 Example 4 987 72.1 680 Example 5 936 73.2 653 Example 6 972 70 661 Example 7 836 72.5 580 Example 8 910 73 600 Comparative 635 55 350 Example 1 Comparative 980 40.8 386 Example 2

Having thus described a method of preparing a selenium carbon composite material, a method of making immobilized selenium and the use of the immobilized selenium, e.g., in a rechargeable battery, will be described.

Selenium is an element in the same group as oxygen and sulfur namely, Group 6 of the Periodic Table of the elements. Selenium may be advantageous over oxygen and sulfur in term of its substantially high electrical conductivity. US 2012/0225352 discloses making Li-selenium and Na-selenium rechargeable batteries, with good capacity and cycling capability. However, a certain level of polyselenide anions shuttle between the cathodes and anodes of such batteries, resulting in additional electrochemical performances that need to be substantially improved for practical uses. Literature relevant to this field includes the following:

-   “Electrode Materials for Rechargeable Batteries”, Ali Aboulmrane and     Khalil Amine, US Patent Application 2012/0225352, Sep. 6, 2012. -   “Lithium-Selenium Secondary Batteries Having non-Flammable     Electrolyte”, Hui He, Bor Z. Jang, Yanbo Wang, and Aruna Zhamu, US     Patent Application 2015/0064575, Mar. 5, 2015. -   “Electrolyte Solution and Sulfur-based or Selenium-based Batteries     including the Electrolyte Solution”, Fang Dai, Mei Cai, Qiangfeng     Xiao, and Li Yang, US Patent Application 2016/0020491, Jan. 21,     2016. -   “A New Class of Lithium and Sodium Rechargeable Batteries Based on     Selenium and Selenium-Sulfur as a Positive Electrode”, Ali     Abouimrane, Damien Dambournet, Kerena W. Chapman, Peter J. Chupa,     Wei Wang, and Khalil Amine, J. Am. Chem. Soc. 2012, 134, 4505-4508. -   “A Free-Standing and Ultralong-life Lithium-Selenium Battery Cathode     Enabled by 3D Mesoporous Carbon/Graphene Hierachical Architecture”,     Kai Han, Zhao Liu, Jingmei Shen, Yuyuan Lin, Fand Dai, and Hongqi     Ye, Adv. Funct. Mater., 2015, 25, 455-463. -   “Micro- and Mesoporous Carbide-Derived Carbon-Selenium Cathodes for     High-Performance Lithium Selenium Batteries”, Jung Tai Lee, Hyea     Kim, Marin Oschatz, Dong-Chan Lee, Feixiang Wu, Huan-Ting Lin,     Bogdan Zdyrko, Wan Il Chao, Stefan Kaskel, and Gleb Yushin, Adv.     Energy Mater. 2014, 1400981. -   “High-Performance Lithium Selenium Battery with Se/Microporous     Carbon Composite Cathode and Carbonate-Based Electrolyte”, Chao Wu,     Lixia Yuan, Zhen Li, Ziqi Yi, Rui Zeng, Yanrong Li, and Yunhui     Huang, Sci. China Mater. 2015, 58, 91-97. -   “Advanced Se—C Nanocomposites: a Bifunctional Electrode Material for     both Li—Se and Li-ion Batteries”, Huan Ye, Ya-Xia Yin, Shuai-Feng     Zhang, and Yu-Guo Guo, J. Mater. Chem. A., May 23, 2014. -   “Lithium Iodide as a Promising Electrolyte Additive for     Lithium-Sulfur Batteries: Mechanisms of Performance Enhancement”,     Feixiang Wu, Jung Tae Lee, Naoki Nitta, Hyea Kim, Oleg Borodin, and     Gleb Yushin, Adv. Mater. 2015, 27, 101-108. -   “A Se/C Composite as Cathode Material for Rechargeable Lithium     Batteries with Good Electrochemical Performance”, Lili Li, Yuyang     Hou, Yaqiong Yang, Minxia Li, Xiaowei Wang, and Yuping Wu, RSC Adv.,     2014, 4, 9086-9091. -   “Elemental Selenium for Electrochemical Energy Storage”, Chun-Peng     Yang, Ya-Xia Yin, and Yu-Guo Guo, J. Phys. Chem. Lett. 2015, 6,     256-266. -   “Selenium@mesoporous Carbon Composite with Superior Lithium and     Sodium Storage Capacity”, Chao Luo, Yunhua Xu, Yujie Zhu, Yihang     Liu, Shiyou Zheng, Ying Liu, Alex Langrock, and Chunsheng Wang,     ACSNANO, Vol. 7, No. 9, 8003-8010.

Also disclosed herein is immobilized selenium comprising selenium and carbon. Immobilized selenium may comprise elemental form selenium or compound form selenium. Selenium may be doped with other element, such as, but not limited to, sulfur. The immobilized selenium enables the localization of elemental selenium atoms which function electrochemically properly without being shuttled between a cathode and an anode of a battery. Immobilization of selenium allows an elemental selenium atom to gain two electrons during a discharge process and to form a selenide anion at the location where the selenium molecule/atom is immobilized. The selenide anion can then give up two electrons during a charging process to form an elemental selenium atom. Therefore, immobilized selenium can work as an electrochemical active agent for a rechargeable battery that has a specific capacity that may be up to a stoichiometric level, can have a Coulombic efficiency that may be ≥95%, ≥98%, or as high as 100%, and can achieve a substantially-improved sustainable cycling capability.

In a battery made with immobilized selenium the electrochemical behaviors of elemental selenium atoms and selenide anions during charging are processes that desirably function properly. Carbon skeletons possessing Sp² carbon-carbon bonds have delocalized electrons distributed over a conjugated six-member-ring aromatic π-bonds across G-band graphene-like local networks that are bounded by D-band carbon. In the presence of an electrical potential, such delocalized electrons may flow with little or no electrical resistance across the carbon skeleton. Selenium immobilization can also compress a carbon skeleton's Sp² carbon-carbon bonds, resulting in stronger carbon-carbon bonds, possibly leading to improved electron conductivity within the carbon skeleton network. At the same time, selenium immobilization may also lead to compression of selenium particles, resulting in stronger selenium-selenium chemical and physical interactions, possibly leading to improved electrical conductivity among immobilized selenium particles. When both carbon-carbon bonds and Se—Se bonds are enhanced due to selenium immobilization, carbon-selenium interactions are also enhanced by the compression in addition to the presence of a stabilized selenium portion to which carbon skeleton can bond. This portion of the selenium may act as an interface layer for a carbon skeleton to successfully immobilize the stabilized selenium portion. Therefore, electrons may flow with a minimal electrical resistance between the carbon skeleton and the immobilized selenium, whereupon the electrochemical charge/discharge processes may function efficiently in a rechargeable battery. This, in turn, allows the rechargeable battery to maintain a near-stoichiometric specific capacity and have the capability of cycling at almost any practical rate with a low level of damage to the electrochemical performance of the battery.

A carbon skeleton may be porous and may be doped with another composition. The pore size distributions of the carbon skeleton may range between sub angstrom to a few microns or to a pore size that a pore size distribution instrument can characterize by using nitrogen, argon, CO₂ or other absorbent as a probing molecule. The porosity of the carbon skeleton may comprise a pore size distribution that peaks in the range of at least one of the following: between sub-angstrom and 1000 angstroms, or between one angstrom and 100 angstroms, or between one angstrom and 50 angstroms, or between one angstrom and 30 angstroms, and or between one angstrom and 20 angstroms. The porosity of the carbon skeleton may further comprise pores having a pore size distribution with more than one peak in the ranges described in the previous statement. Immobilized selenium may favor carbon skeleton having small pore sizes in which electrons may be delivered and harvested quickly with minimum electrical resistance, which may allow the selenium to function more properly electrochemically in a rechargeable battery. The small pore size may also provide more carbon skeleton surface area where the first portion of the selenium can form a first interface layer for a second portion of selenium immobilization. In addition, the presence in a carbon skeleton having a certain portion of medium size pores and a certain portion of large size pores may also be beneficial for effective delivery of solvent lithium ions from bulk solvent media to a small pore region where lithium ions may lose coordinated solvent molecules and get transported in solid phase of lithium selenide.

The pore volume of the carbon skeleton may be as low as 0.01 mL/g and may be as much as 5 mL/g, or may be between 0.01 mL/g and 3 mL/g, or may be between 0.03 mL/g and 2.5 mL/g, or may be between 0.05 mL/g and 2.0 mL/g. The pore volume having pore sizes less than 100 angstroms, or less than 50 angstrom, or less than 30 angstroms, or less than 20 angstroms may be greater than 30%, or greater than 40%, or greater than 50%, or greater than 60%, or greater than 70%, or greater than 80% of the total measurable pore volume that can be measured by using a BET instrument with nitrogen, CO₂, argon, and other probing gas molecules. The BET determined surface area of the carbon may be greater than 400 m²/g, or greater than 500 m²/g, or greater than 600 m²/g, or greater than 700 m²/g, or greater than 800 m²/g, or greater than 900 m²/g, or greater than 1000 m²/g.

The carbon may also be substantially amorphous, or it may have a characteristic of a very broad peak centered at a d-spacing around 5 angstroms.

The carbon may comprise Sp² carbon-carbon bonds, having Raman peak shifts featuring a D-band and a G-band. In an example, Sp² carbon-carbon bonds of the carbon feature a D-band centered at 1364±100 cm⁻¹ with a FWHM about 296±50 cm⁻¹ and a G-band center at 1589±100 cm⁻¹ with a FWHM about 96±50 cm⁻¹ in Raman spectrum. The ratio of the area of D-band to G-band may range from 0.01 to 100, or from 0.1 to 50, or from 0.2 and 20.

The carbon may be of any morphology, namely, for example, platelet, sphere, fiber, needle, tubular, irregular, interconnected, agglomerated, discrete, or any solid particles. Platelet, fiber, needle, tubular, or some morphology having a certain level of aspect ratio may be beneficial for achieving better inter-particle contact, resulting in better electrical conductivity, possibly enhancing rechargeable battery performance.

The carbon may be of any particle size, having a median particle size from a nanometer to a few millimeters, or from a few nanometers to less than 1000 microns, or from 20 nm to 100 microns.

The property of a carbon skeleton can affect selenium immobilization and interactions between the carbon skeleton surface and selenium particles can affect the performance of a rechargeable battery. The location of Sp² carbon in a carbon skeleton can aid in achieving Se immobilization. Sp² carbon from small carbon skeleton pores may be favored, which can be quantified by NLDFT surface area method, as discussed in the Example 9 herein. The surface area from carbon skeleton pores less than 20 angstroms may be ≥500 m²/g, ≥600 m²/g, 700 m²/g, ≥800 m²/g, ≥900 m²/g, or ≥1,000 m²/g. The surface areas from the carbon skeleton pores between 20 angstroms and 1000 angstroms may be 20% or less, 15% or less, 10% or less, 5% or less, 3% or less, 2% or less, or 1% or less of the total surface area of the carbon skeleton.

Immobilized selenium can comprise selenium that vaporizes at a temperature higher than elemental selenium, referring to the following definition of selenium vaporization: Elemental selenium in a Se-Super P composite loses 50% of its weight at a temperature of 480° C.; elemental selenium in a Se/Graphite composite loses its weight by 50% of the contained selenium at a temperature of 471° C. Immobilized selenium loses 50% of its weight at a temperature higher than 480° C., for example at a temperature ≥490° C., ≥500° C., ≥510° C., ≥520° C., ≥530° C., ≥540° C., ≥550° C., ≥560° C., ≥570° C., or ≥580° C. or more. Selenium in the immobilized selenium may need a kinetic energy of ≥9.5 kJ/mole, ≥9.7 kJ/mole, ≥9.9 kJ/mole, ≥10.1 kJ/mole, ≥10.3 kJ/mole, or ≥10.5 kJ/mole or more to overcome the bonding and or intermolecular forces in the immobilized selenium system and to escape to the gas phase. In an example, the last portion of the immobilized selenium that vaporizes can require a kinetic energy of ≥11,635 joules/mole (≥660° C.) to escape the carbon skeleton and may be critical for selenium immobilization and may work as interfacial material between the carbon skeleton and the majority of the immobilized selenium molecules. Therefore, this portion of the selenium that requires a kinetic energy of ≥11,635 joules/mole is called interfacial selenium. The amount of interfacial selenium in the immobilized selenium may be ≥1.5%. ≥2.0%, ≥2.5%, or 3.0% of the total immobilized selenium.

Immobilized selenium can comprise selenium that has an activation energy higher than that for conventional (non-immobilized) selenium to overcome in order for the selenium to escape from the immobilized Se—C composite system. The activation energy for non-immobilized selenium (Se-Super P composite system) was determined to be about 92 kJ/mole according to ASTM Method E1641-16. The activation energy for selenium in the immobilized selenium comprising selenium and carbon is ≥95 kJ/mole, ≥98 kJ/mole, ≥101 kJ/mole, ≥104 kJ/mole, ≥107 kJ/mole, or ≥110 kJ/mole. The activation energy for selenium in the immobilized selenium comprising selenium and carbon is ≥3%, ≥6%, ≥9%, ≥12%, ≥15%, or ≥18% greater than that for selenium in Se-Super P composite. The immobilized selenium can be more stable than non-immobilized selenium, which is the reason that the battery comprising immobilized selenium may cycle electrochemically better, probably due to the minimization (or elimination) of selenium shuttling between cathode and anode, resulting from selenium being immobilized in Se—C composite.

Immobilized selenium may comprise selenium that may be Raman-inactive or Raman-active, typically having a Raman peak at 255±25 cm⁻¹, or at 255±15 cm⁻¹, or at 255±10 cm⁻¹. Raman relative peak intensity is defined as the area of the Raman peak at 255 cm⁻¹ relative to the area of the D-band peak of the carbon Raman spectrum. Immobilized carbon may comprise selenium having a Raman relative peak intensity of ≥0.1%, ≥0.5%, ≥1%, ≥3%, ≥5%. Immobilized selenium may contain ≥5% selenium, ≥10% selenium, ≥20% selenium, ≥30% selenium, ≥40% selenium, ≥50% selenium, ≥60% selenium, or ≥70% selenium.

Immobilized selenium can comprise selenium having a red shift from the Raman peak of pure selenium. A red shift is defined by a positive difference between the Raman peak location for the immobilized selenium and that for pure selenium. Pure selenium typically has a Raman peak at about 235 cm⁻¹. Immobilized selenium can comprise selenium that has a red shift of the Raman peak by ≥4 cm⁻¹, ≥6 cm⁻¹, ≥8 cm⁻¹, ≥10 cm⁻¹, ≥12 cm⁻¹, ≥14 cm⁻¹, or ≥16 cm⁻¹. A red shift in Raman peak suggests that there is a compression on the selenium particles.

Immobilized selenium can comprise carbon that may be under compression. Under compression, electrons can flow with a minimum resistance, which facilitates fast electron delivery to selenium and from selenium anions for electrochemical processes during discharge-charge processes for a rechargeable battery. D-band and or G-band in Raman spectrum for the Sp² carbon-carbon bonds of the carbon skeleton comprising the immobilized selenium may show a red shift, by ≥1 cm⁻¹, ≥2 cm⁻¹, ≥3 cm⁻¹, ≥4 cm⁻¹, or ≥5 cm⁻¹.

Immobilized selenium comprises selenium that can have a higher collision frequency than non-immobilized selenium. Such high collision frequency may result from selenium in the immobilized Se—C system that is under compression. The collision frequency for selenium in non-immobilized selenium was determined to be around 2.27×10⁵, according to the ATSM Method E1641-16. The collision frequency for selenium in the immobilized selenium comprising selenium and carbon is ≥2.5×10⁵≥3.0×10⁵≥3.5×10⁵, ≥4.0×10⁵≥4.5×10⁵, ≥5.0×10⁵≥5.5×10⁵≥6.0×10⁵, or ≥8.0×10⁵. The immobilized selenium can have a higher collision frequency by ≥10%, 30%, 50%, 80%, 100%, 130%, 150%, 180%, or ≥200% than that for non-immobilized selenium in Se—C composite. This may lead to better electron conductivity in the immobilized selenium system because of more collisions among selenium species. The immobilized selenium in Se—C composite would also have a higher collision frequency against the wall of the carbon host (e.g., a carbon skeleton), which may result in a better delivery or harvesting of electrons from the carbon skeleton during electrochemical cycling, which can lead to a battery (comprising immobilized selenium) that has improved cycling performances, such as attaining more cycles and or cycling at a much higher C-rate, which is highly desirable.

Immobilized selenium comprises selenium that has less tendency to leave its host material (carbon), having a kinetic rate constant that is ≤⅕, ≤ 1/10, ≤ 1/50, ≤ 1/100, ≤ 1/500, or ≤ 1/1000 of the kinetic rate constant for non-immobilized/conventional selenium. In our example, immobilized selenium comprises selenium that has less tendency to leave its host material (carbon), having a kinetic rate constant (at 50° C.) of ≤1×10⁻¹⁰, ≤5×10⁻¹¹, 1×10⁻¹¹, ≤5×10⁻¹², or ≤5×10⁻¹³.

Immobilized selenium can comprise selenium that may be amorphous, as determined by X-ray diffraction measurements. A diffraction peak having a d-spacing of about 5.2 angstroms is relatively smaller or weaker, for example, 10% weaker, 20% weaker, 30% weaker, or 40% weaker, than that for the carbon skeleton.

Immobilized selenium may be prepared by physically mixing carbon and selenium followed by melting and homogenizing (or mixing or blending) selenium molecules to achieve selenium immobilization. The physical mixing may be achieved by ball-milling (dry and wet), mixing with mortar and pestle (dry or wet), jet-milling, horizontal milling, attrition milling, high shear mixing in slurries, regular slurry mixing with blade, etc. The physically mixed mixture of selenium and carbon may be heated at a temperature that is at or higher than the melting point of selenium and below the melting temperature of carbon. The heating may be carried out in an inert gas environment such as, but not limited to, argon, helium, nitrogen, etc. The heating environment may comprise air or a reactive environment. Immobilization of selenium may be achieved by impregnating dissolved selenium into carbon, followed by evaporation of the solvent. The solvent for dissolving selenium may comprise an alcohol, an ether, an ester, a ketone, a hydrocarbon, a halogenated hydrocarbon, a nitrogen-containing compound, a phosphorus containing compound, a sulfur-containing compound, water, etc.

Immobilized selenium can be achieved by melting a large amount of selenium in the presence of carbon, followed by removing extra non-immobilized selenium.

An immobilized selenium system or body may comprise immobilized selenium ≥30%, ≥40%, ≥50%, ≥60%, ≥70%, ≥80%, or ≥90% of the total amount of selenium in the system or body. The non-immobilized selenium can vaporize at a temperature lower than the immobilized selenium.

An immobilized selenium system or body may comprise immobilized selenium that is doped with one or more additional/other element(s) from Group 6 of the Periodic Table, such as, for example, sulfur and/or tellurium. The dopant level may range from as low as 100 ppm by weight to as high as 85% of the weight of the immobilized selenium system or body.

An example process of making immobilized selenium is illustrated in FIG. 6. In the process, selenium and carbon are mixed together (S1) under dry or wet conditions. The mixture can be optionally dried to a powder (S2) followed by optionally pelletizing the dried powder (S3). The results of step S and optionally steps S2 and S3 produce a carbon skeleton that is a starting material for step S4. In step S4, selenium is melted into the carbon skeleton. The selenium melted into the carbon skeleton is allowed to dry thereby producing the immobilized selenium of step S5. Preparation and characterization of immobilized selenium will be described later herein in connection with Example 10.

The immobilized selenium may be used as a cathode material for a rechargeable battery. For making a cathode, the immobilized selenium may be dispersed in a liquid media such as, but not limited to, water or an organic solvent. The cathode comprising the immobilized selenium may comprise a binder, optionally another binder, optionally an electric-conductivity promoter, and an electric charge collector. The binder may be an inorganic or organic. An organic binder may be of a natural product, such as, for example, CMC, or a synthetic product, such as, for example, a SBR Rubber latex. An electrical-conductivity promoter can be a type of carbon, such as, graphite-derived small particles, graphene, carbon nano-tubes, carbon nano-sheet, carbon blacks, etc. An electric charge collector may be, for example, an aluminum foil, a copper foil, a carbon foil, a carbon fabric, or other metallic foils. The cathode can be prepared by coating an immobilized selenium-containing slurry (or slurries) onto the charge collector, followed by a typical drying process (air dry, oven-dry, vacuum oven-dry, etc.). The immobilized selenium slurry or slurries may be prepared by a high shear mixer, a regular mixer, a planetary mixer, a double-planetary mixer, a ball mill, a vertical attritor, a horizontal mill, etc. The cathode comprising immobilized selenium may be pressed or roller-milled (or calendared) prior to its use in a battery assembly.

A rechargeable battery comprising immobilized selenium may comprise a cathode comprising immobilized selenium, an anode, a separator, and an electrolyte. The anode may comprise lithium, sodium, silicon, graphite, magnesium, tin, and/or and suitable and/or desirable element or combination of elements from Group IA, Group IIA, Group IIIA, etc., of the periodic table of the elements (Periodic Table). The separator may comprise an organic separator, an inorganic separator, or a solid electrolyte separator. An organic separator may comprise a polymer such as, for example, polyethylene, polypropylene, polyester, a halogenated polymer, a polyether, a polyketone, etc. An inorganic separator may comprise a glass or quartz fiber, a solid electrolyte separator. An electrolyte may comprise a lithium salt, a sodium salt, or other salt, a salt of Group 1A of the Periodic Table, a salt of Group IIA of the Periodic Table, and an organic solvent. The organic solvent may comprise an organic carbonate compound, an ether, an alcohol, an ester, a hydrocarbon, a halogenated hydrocarbon, a lithium containing-solvent, etc.

A rechargeable battery comprising immobilized selenium may be used for electronics, an electric or hybrid vehicle, an industrial application, a military application such as a drone, an aerospace application, a marine application, etc.

A rechargeable battery comprising immobilized selenium may have a specific capacity of 400 mAh/g active amount of selenium or higher, 450 mAh/g or higher, 500 mAh/g or higher, 550 mAh/g or higher, or 600 mAh/g or higher. A rechargeable battery comprising immobilized selenium may be able to undergo electrochemical cycling for 50 cycle or more, 75 cycles or more, 100 cycles or more, 200 cycles or more, etc.

A rechargeable battery comprising immobilized selenium may be able to be charged at 0.1 C, 0.2 C, 0.5 C, 1 C, 1.5 C, 2 C, 3 C, 4 C, 5 C, 6 C, 7 C, 8 C, 9 C, 10 C or faster. After conducting extensive high C-Rate charge-discharge cycling for 30 or more cycles (e.g., 5 cycles at 0.1 C, 5 cycles at 0.2 C, 5 cycles at 0.5 C, 5 cycles at 1 C, 5 cycles at 2 C, 5 cycles at 5 C, and 5 cycles at 10 C), a rechargeable battery comprising immobilized selenium may retain a battery specific capacity >30%, >40%, >50%, >60%, >70%, >80% of the 2^(nd) discharge specific capacity at a cycling rate of 0.1 C.

Following are several examples to illustrate the spirit of the inventions. However, these examples should not be construed in a limiting sense.

Examples

Method of Characterization

Scanning Electron Microscopy (SEM) images were collected on a Tescan Vega scanning electron microscope equipped with an energy dispersive analysis X-Ray (EDX) detector.

Raman spectra were collected by a Renishaw inVia Raman Microscope (confocal). Laser Raman spectroscopy is widely used as a standard for the characterization of carbon and diamond and provides readily distinguishable signatures of each of the different forms (allotropes) of carbon (e.g., diamond, graphite, buckyballs, etc.). Combined with photoluminescence (PL) technology, it offers a non-destructive way to study various properties of diamond including phase purity, crystal size and orientation, defect level and structure, impurity type and concentration, and stress and strain. In particular, the width (full-width-half-maximum, FWHM) of the first order diamond Raman peak at 1332 cm⁻¹, as well as the Raman intensity ratio between diamond peak and graphitic peaks (D-band at 1350 cm⁻¹ and G-band at 1600 cm⁻¹), is a direct indicator of diamond and other carbon material quality. Furthermore, the stress and strain levels in diamond or other carbon grains and films can be estimated from diamond Raman peak shift. It has been reported that the diamond Raman peak shift rate under hydrostatic stress is about 3.2 cm⁻¹/GPa, with the peak shifting to lower wavenumber under tensile stress and higher wavenumber under compressive stress. The Raman spectra discussed herein were collected using a Renishaw inVia Raman spectroscope with 514 nm excitation laser. More information about using Raman spectroscopy to characterize diamond is also available in the references (1) A. M. Zaitsev, Optical Properties of Diamond, 2001, Springer and (2) S. Prawer, R. J. Nemanich, Phil. Trans. R. Soc. Lond. A (2004) 362, 2537-2565.

The data for BET surface area and pore size distributions of carbon samples were measured by nitrogen absorption and CO₂ absorption with a 3Flex (Mircomeritics) equipped with a Smart VacPrep for sample degas preparations. The sample is typically degased in Smart Vac-Prep at 250° C. for 2 hours under vacuum prior to CO₂ and N₂ absorption measurements. Nitrogen absorption is used to determine the BET surface area. Nitrogen absorption data and CO₂ absorption data were combined to calculate pore size distributions of a carbon sample. For the details about combining both N₂ and CO₂ absorption data for determining the pore size distributions for carbon materials, please refer to “Dual gas analysis of microporous carbon using 2D-NLDFT heterogeneous surface model and combined adsorption data of N₂ and CO₂”, Jacek Jagiello, Conchi Ania, Jose B. Parra, and Cameron Cook, Carbon 91, 2015, page 330-337.

The data for thermogravimetric analysis (TGA) and TGA-differential scanning calorimetry (DSC) for immobilized selenium samples and the control samples were measured by Netzsch Thermal Analyzer. The TGA analysis was performed under an argon flow rate of ˜200 mLmin at a heating rate of 16° C./min, 10° C./min, 5° C./min, 2° C./min, 1° C./min, and other heating rates. For the purpose of consistency, a typical amount of immobilized selenium sample used for TGA analysis was about 20 mg.

Activation energy and collision frequency of the immobilized selenium and non-immobilized selenium were determined by TGA following the procedures described in ASTM Method E1641-16.

X-Ray diffraction results for different carbon, Se-carbon samples, and immobilized selenium were collected on a Philip Diffractometer.

Battery cycling performances for rechargeable batteries comprising immobilized selenium were tested on Lanhe CT2001A Battery Cycling Tester. Charge and discharge currents of the rechargeable batteries comprising immobilized selenium were determined by the amount of selenium contained in the immobilized selenium and cycling rate (0.1 C, 0.5 C, 1 C, 2 C, 3 C, 4 C, 5 C, 10 C, etc.).

Example 9: Synthesis and Characterization of Carbon Skeleton

To form a first residue, a charge of 260 g potassium citrate was included in a crucible and the crucible was placed into a quartz tubing inside a tubular furnace. A stream of argon gas was flowed into the furnace and the furnace was heated at 5° C./min from room temperature (−20-22° C.) to 600° C. The furnace was held at this temperature for 60 minutes, followed by shutting-down the furnaces and removing the charge from the crucible after furnace cooling down, recovering 174.10 grams of processed residue. To form second and third processed residues, the same process described for the first residue was repeated for charges of 420 and 974 grams of potassium citrate, separately. The resulting second and third processed residues weighed 282.83 grams and 651.93 grams, respectively.

1108.9 grams from these three processed residues were combined together into a crucible, which was placed into the quartz tubing inside the tubular furnace and a flow of argon gas was streamed into the furnace. The furnace was heated at 5° C./min to 800° C. The furnace was held at 800° C. for 1 hour. The furnace was allowed to cool whereupon the crucible was removed from the quartz tubing and thereafter 1085.74 grams of a first final residue were recovered.

Following the same procedure described in this Example (800° C.), a charge of 120 grams of potassium residues introduced into the furnace produced about 77 grams of a second final residue (800° C.).

The combination of the first and second final residues resulted in about 1,163 grams of a third final residue.

The 1,163 grams of third final residue was then mixed with 400 ml of water to form a slurry which was separated approximately equally into four two-liter beakers. The pH of each slurry was measured to be greater than 13. Next, a concentrated hydrochloric acid solution was added to each beaker with a violent evolution of carbon dioxide, which subsided at a pH less than about 5. More hydrochloric acid solution was added to obtain a pH of about 1.9. Then the slurries were filtered and washed to filter cakes that were dried in an oven at 120° C. for about 12 hours, followed by vacuum drying at 120° C. for 24 hours resulting in four carbon skeleton samples, a total of about 61.07 grams.

These carbon skeleton samples were characterized with SEM, XRD, Raman, BET/Pore-Size-Distributions. The SEM result for one carbon skeleton is shown FIG. 7. Surface morphologies of typical carbon skeleton particles that are prepared in the process described in this Example, had sheet-like morphologies with their sheet edges being interconnected with sample thickness between 500 nm and 100 nm, and the sample width (or length) being between 0.5 and 2 μm, therefore having an aspect ratio (defined as the ratio of the longest dimension of the sample width (or sheet length) to the sample thickness) ≥1, e.g., an aspect ratio ≥5 or greater, or an aspect ratio ≥10.

X-Ray diffraction patterns of one carbon skeleton, shown in FIG. 8, shows that the carbon skeleton is substantially amorphous in phase. However, it does show a broad diffraction peak centered at around 2θ of about 17°, indicating a d-spacing about 5.21 angstroms.

Raman scattering spectroscopy results for one carbon skeleton is shown in FIG. 9, showing Sp² carbon having a D-band at about 1365 cm⁻¹ (Curve 1) and G-band at about 1589 cm⁻¹ (Curve 2) with a FWHM of 296 and 96 cm⁻¹, respectively. Both D-band and G-band show a mixture of Gaussian and Lorentian distributions; D-band has about 33% Gaussian distributions and G-band has about 61% Gaussian distributions. The ratio of the area for D-band to the area for G-band is about 3.5.

BET surface area of one carbon skeleton was measured to be 1,205 m²/g by nitrogen absorption. The Incremental Pore Surface Area vs. the pore width is plotted in FIG. 10A by using NLDFT method, showing a Cumulative Pore Surface Area of 1,515 m²/g. The discrepancy between BET surface area and NLDFT surface area may come from the fact that NLDFT distributions are calculated with both nitrogen and CO₂ absorption data; CO₂ molecules may enter the pores smaller than those pores that nitrogen molecules can enter. The NLDFT surface area in the pores peaked at 3.48 angstrom is 443 m²/g, at 5.33 angstrom is 859 m²/g, and at 11.86 angstrom (up to 20 angstroms) is 185 m²/g, a total of 1,502 m²/g for the pores of 20 angstroms or smaller, while the NLDFT surface area from the pores between 20 angstrom and 1000 angstrom is only 7.5 m²/g, and the surface area from the pores of 20 angstroms or greater is only about 0.5% of the total surface area.

The pore size distributions of the carbon skeleton sample were determined by nitrogen absorption and CO₂ absorption. The absorption results from nitrogen absorption and CO₂ absorption were combined to produce the pore-size-distribution shown in FIG. 10B. The relationship of Incremental Pore Volume (mL/g) vs. the pore width (angstrom) shows that there are three major peaks located at 3.66 angstroms, 5.33 angstroms, and 11.86 angstroms; the relationship of the Cumulative Pore Volume (mL/g) vs. pore width (angstrom) shows that there are about 0.080 mL/g pores under the peak of 3.66 angstroms, about 0.240 mL/g pores under the peak of 5.33 angstroms, about 0.108 mL/g pores under the peak of 11.86 angstroms, having 0.43 mL/g pores of 20 angstroms or smaller, 0.042 mL/g pores between 20 and 100 angstroms, and a total of 0.572 mL/g pores of up to 1000 angstroms.

Example 10: Preparation and Characterization of Immobilized Selenium

Include 0.1206 grams of selenium (showing bulk properties of selenium) into a set of agate mortar and pestle and include 0.1206 grams of the carbon skeleton that was prepared in accordance with Example 9 into the same agate mortar and pestle. Manually grind the mixture of selenium and carbon skeleton for about 30 minutes and transfer the ground mixture of selenium and carbon skeleton into a stainless steel die (10 mm in diameter). Press the mixture in the die to a pressure of about 10 MPa to form a pellet of the mixture. Then, load the pellet into a sealed container in the presence of an inert environment (argon) and place the sealed container containing the pellet into an oven. Heat the oven including the sealed container containing the pellet to 240° C. (above the melting temperature of selenium) for, for example, 12 hours. Use, however, is envisioned of any combination of time and temperature, above the melting temperature of selenium, sufficient to cause the selenium and carbon to react, either partially or fully react, and form immobilized selenium having some or all of the features described in this application. Next, unload the pellet from the container after allowing the pellet to return to room temperature. The unloaded pellet is the immobilized selenium of this Example 10.

The immobilized selenium of this Example 10 was then characterized by TGA-DSC and TGA. TGA-DSC analysis results were collected for the immobilized selenium under a stream of 200 ml/min of argon gas at a heating rate of 10 C/min. There is no observable endothermic DSC peak at temperatures near the melting point of selenium (about 230° C.), indicating that the immobilized selenium of this Example 10 is different from the bulk-form of selenium molecules/atoms which should have a melting point at around 230° C. where there should be a endothermic peak.

An investigation revealed that the TGA-DSC data may not be reliable when the heating temperature reaches a point where selenium molecules start to escape from the TGA-DSC sample crucible (graphite or ceramics). To this end, gas phase selenium molecules (from the sample crucible) enter the argon carrier gas stream and appear to react with the TGA-DSC platinum sample holder, which distorts the actual TGA-DSC thermochemical behaviors. The released selenium molecules from sample crucible reacting with platinum sample holder, lead to a lower weight loss in this temperature region. The selenium-platinum composite in the platinum sample holder is then released into the gas phase when the heating temperature reaches a point that is beyond 800° C. A complete selenium release can occur at 1000° C. This investigation used up most of the immobilized selenium sample of this Example 10. Therefore, a new sample of the immobilized selenium (−16 grams) was prepared using the same process as was described in the earlier part of this Example 10.

The thermochemical behaviors of this new sample of immobilized selenium were studied by TGA analysis, which uses a ceramic sample holder that covers a very small thermocouple that is used for the TGA analysis. The TGA analysis results for this new immobilized selenium sample are shown in FIG. 11A along with the TGA analysis results (FIG. 11B) for selenium-carbon composites (made with 50-50 Se-Super P carbon composite, and Se-Graphite (ground graphite), in the same process as preparation of the immobilized selenium of this Example 10. Super P is a commercial grade carbon black widely used for lithium-ion battery industry. Ground graphite was prepared by grinding Poco 2020 graphite. The TGA analysis data are also summarized in the following Table 2.

TABLE 2 Se- Immobilized Se in Example 10 Graphite Se-Super Temp. at the End Temp. Comp. P Comp. bottom of the of TGA Mid-Weight-Loss Temperature main wt. loss Expt. Mid-Wt.-Loss 471 480 595 660 1000 Temp, ° C. Mid-Wt.-Loss 744 753 868 933 1273 Temp, K Kinetic Energy, 9,278 9,391 10,825 11,635 15,876 Joule/Mole

Immobilized selenium can have an initial weight loss temperature starting at about 400° C. vs. 340° C. for Se-Super P carbon composite and the Se-Graphite carbon composite; a mid-point weight-loss temperature for immobilized selenium can be at about 595° C. vs. 480° C. for the Se-Super P composite and 471° C. for Se-Graphite composite; and main weight loss completed at about 544° C. for Se-Super P composite and Se-Graphite composite, and 660° C. for the immobilized selenium. The Se-Super P carbon composite and Se-Graphite carbon composite show less than 0.6% weight loss between 560° C. and 780° C., while immobilized selenium shows a weight loss of about 2.5% from the bottom of the main weight loss (˜660° C. to 1000° C.). These results suggest that non-immobilized selenium (Se-Super P carbon composite and Se-Graphite composite) has ≤1.2% of the total selenium which can escape from the composite at a temperature of ≥560° C., while the immobilized selenium has about 5.0% of the total selenium which can escape from carbon skeleton at a temperature of ≥660° C. The following details are provided to give examples that provide insight to the thermochemical behaviors. However, these details are not to be construed in a limiting sense.

Using the data of TGA mid-weight-loss temperature as examples of thermochemical behaviors, as the heating temperature increases, the kinetic energy of the selenium molecules in Se-Super P composite and Se-Graphite composite increase to a level at which these selenium molecules have enough energy to overcome the intermolecular interactions among selenium molecules and escape from liquid phase of the selenium. Herein Kinetic Energy=3RT/2, wherein: R is gas constant and T is temperature in Kelvin.

It was observed that the average kinetic energy of selenium molecules for Se-Super P composite was measured to be 9,391 joules/mole when the selenium molecules escape from the mixture of Se-Super P composite. However, the immobilized selenium needs to gain more energy to have an average kinetic energy of about 10,825 joules/mole for selenium to leave the carbon skeleton to gas phase selenium molecules. It is believed that the selenium in immobilized selenium, either as an atomic form, as a molecular form, or as any form, may chemically interact with selenium and the carbon skeleton beyond intermolecular interactions of selenium. In addition, the last portion of selenium that escapes from the carbon skeleton between 660° C. to 1000° C. has an average kinetic energy in the range from 11,635 joules/mole to 15,876 joules/moles or more. This suggests that selenium in the immobilized selenium is more stable than the selenium in conventional selenium-carbon composites. The stabilized selenium in the immobilized selenium of this Example 10 enhances the ability of selenium, either as atomic forms, as molecular forms, or in any forms, to stay inside the carbon skeleton during electrochemical processes, such as during charge and discharge cycling of a rechargeable battery comprised of the immobilized selenium. In an example, this last portion of selenium can require a kinetic energy of ≥11,635 joules/mole (≥660° C.) to escape the carbon skeleton and may be critical for selenium immobilization and may work as interfacial material between carbon skeleton and the majority of the immobilized selenium molecules. The portion of interfacial selenium in the immobilized selenium may be ≥1.5%. ≥2.0%. ≥2.5%. or 3.0% of the total immobilized selenium.

FIG. 11A also shows the TGA studies of immobilized selenium with a heating rate of 16° C./min, having a temperature of 628° C. for the mid-point-weight-loss of the contained selenium. As shown in FIG. 11C, for Se-Super P composite, the temperature at the mid-weight-loss of the contained Se at a heating rate of 16° C./min is at 495° C. With different heating rates (e.g., 16° C./min, 10° C./min, 5° C./min, 2.5° C./min, and 1° C./min), activation energy and collision frequency may be determined and calculated using known methods, such as ASTM E1641-16 and E2958-14. The temperatures at 15% weight loss for different heating rates are tabulated as shown in the following Table 3.

TABLE 3 Temperature (° C.) Immobilized Immobilized Se prepared Se prepared by the process by the process of Example 10 of Example 10 Se-Super β (° C./min) (228-110) (155-82-2) P Composite 16 590.65 570.13 471.08 10 560.82 544.86 456.61 5 535.57 515.09 413.37 2.5 506.66 493.27 397.21 1 478.48 462.18 365.02 Activation 120.7 120.0 92.3 Energy, kJ/mole Frequency of 12.4 × 10⁵ 18.3 × 10⁵ 2.27 × 10⁵ Collisions

The activation energy for selenium (non-immobilized or conventional) in the Se-Super P composite was determined to be 92.3/mole with a frequency of collisions at 2.27×10⁵. The activation energy for selenium in immobilized selenium (228-110 above) was also determined to be 120.7 kJ/mole with a frequency of collisions at 12.4×10⁵. Another sample of immobilized selenium (155-82-2 above) that was prepared in the same procedures as Example 10 was also measured to have an activation energy of 120.0 kJ/mole and a frequency of collisions at 18.3×10⁵.

The kinetic rate constant for selenium is calculated using the Arrhenius equation

k=Ae^(−E) ^(a) ^(/RT)

where k is the rate constant, E_(a) is the activation energy, A is frequency of collisions, R is the gas constant, and T is the temperature in Kelvin.

Referring to FIG. 11D, with above determined activation energy and collision frequency, the kinetic rate constant was calculated using the Arrhenius equation at different temperatures. FIG. 11D shows that non-immobilized selenium (Se-Super P composite-solid line) has much higher rate constant than that for immobilized selenium (228-110 (dotted line) and 115-82-2 (dashed line)), for example, about four orders of magnitude greater at 35° C. and about three orders of magnitude greater at 100° C. In an example, at 50° C., the rate constant for non-immobilized selenium (Super P) is 2.668×10-10 while immobilized selenium has a rate constant at 7.26×10⁻¹⁴ (155-82-2) and 3.78×10⁻¹⁴ (228-110). Selenium that has a lower kinetic rate constant has less tendency to leave the host material (carbon), which may lead to better battery cycling performance.

FIG. 12 shows the spectrum of the immobilized selenium with Raman peaks of the D-band at 1368 cm⁻¹ and the G-band at 1596 cm⁻¹, having a ratio of the area for the D-band to the area for the G-band at 2.8. As compared to the Raman spectrum of the carbon skeleton shown in FIG. 9, selenium immobilization shifts both Raman peaks to a higher wavenumber, about 3 cm⁻¹ red shift for the D-band and 7 cm⁻¹ red shift for the G-band, which suggests that the bonding strength of Sp² carbon in the carbon skeleton is being strengthened, with a red shift of about 4 cm⁻¹ for the D-Band and a red shift of about 8 cm⁻¹ for the G-band. At the same time, the ratio of the area for the D-band to the area for the G-band was also decreased from about 3.4 to 2.8, suggesting that either the D-band gets relatively weaker or G-band gets relatively stronger. A stronger G-band may be desirable since the G-band can relate to a type of carbon that allows the carbon skeleton to more readily conduct electrons, which can be desirable for electrochemical performances when used in a rechargeable battery. Bulk or pure selenium typically shows a sharp Raman shift peak at about 235 cm⁻¹. For immobilized selenium, the Raman spectrum in FIG. 12 shows a broad Raman peak at about 257 cm⁻¹ (−12.7% of the G-band in area) and a new broad hump at about 500 cm⁻¹ (about 3.0% of the G-band area). It is believed that selenium immobilization changes Raman characteristics for both carbon skeleton and selenium, with all Raman peaks shifted to a higher wavenumber, suggesting that both of carbon-carbon Sp² bonds for the carbon skeleton and selenium-selenium bonds of the selenium are under compression.

The compression resulting from selenium immobilization strengthens both carbon-carbon Sp² bonds for carbon skeleton and Se—Se bonds for selenium, creating stronger selenium-selenium and carbon-selenium interactions. Therefore, more kinetic energy would be needed for selenium to overcome the stronger Se—Se bonding and stronger carbon-selenium interactions, which explains the observations in TGA analysis of the immobilized selenium vs. Se-Super P composite and Se-Graphite composite.

Furthermore, under compression, the carbon skeleton would then have a better capability of conducting electrons at the bonding level; and under compression, selenium atoms or molecules would also have better capability of conducting electrons.

Stabilized selenium for the immobilized selenium along with enhanced electron conductivity across the carbon skeleton and selenium can be desirable in electrochemical processes, such as, for example, improved specific capacity for the active material with a minimum level of shuttling, improved cycling capability due to the immobilization, a capability of being charged and discharged at a higher rate, etc. However, this is not to be construed in a limiting sense.

X-ray diffraction patterns for the immobilized selenium prepared in accordance with Example 10, shown in FIG. 13, show a decrease in the intensity of the broad diffraction peak from the carbon skeleton with a d-spacing at about 5.21 angstroms—only about ⅓ the intensity, suggesting that immobilized selenium further makes the carbon skeleton more disordered, or causes more destruction to the order of the carbon skeleton. In an example, it is believed that this is because the compression forces are applied on the carbon-carbon Sp² bonds.

FIG. 14 shows SEM image for the immobilized selenium that was prepared in accordance with Example 10, showing sheet-like morphologies, just like the image in FIG. 7 for carbon skeleton. Though there is about 50% selenium that was immobilized in the carbon skeleton, there is no observable selenium particles on the surfaces of the carbon skeleton, except that the inter-sheet connections have been destroyed, resulting in many flat sheets having high aspect ratios. These sheet-like morphologies can be highly desirable for forming oriented coating aligned along the flat-sheet directions, creating sheet surface to surface contacts, leading to improved inter-sheet electrical conductivity, which may result in superior electrical performance for electrochemical processes, such as in a rechargeable battery.

Example 11: Se Cathode Preparation

Into a mortar and pestle include 56 mg of the immobilized selenium that was prepared in accordance with Example 10; 7.04 mg of Super P; 182 μL of carboxymethyl cellulose (CMC) solution (which includes 1 mg of dry CMC for every 52 μL of CMC solution); 21.126 μL of SBR Latex dispersion (which contains 1 mg dry SBR Latex for every 6.036 μL SBR Latex dispersion); and 200 μL deionized water. Grind the particles, the binders, and water manually into a slurry for 30 minutes to produce a cathode slurry. The cathode slurry was then coated onto one-side of a piece of an electrically conductive substrate, e.g., a foil, and air-dried. In an example, the conductive substrate or foil can be an aluminum (Al) foil. However this is not to be construed in a limiting sense since use of any suitable and/or desirable electrically conductive material of any shape or form, is envisioned. For the purpose of description only, the use of Al foil to form a selenium cathode will be described herein. However this is not to be construed in a limiting sense.

The slurry coated Al foil was then placed into a drying oven and heated to a temperature of 55° C. for 12 hours, resulting in a selenium cathode comprised of a dried sheet of immobilized selenium on one side of the Al foil, with the other side of the Al foil being uncoated, i.e., bare aluminum.

The selenium cathode was then punched to cathode discs, each having a diameter of 10 mm. Some of these cathode discs were used as cathodes for rechargeable batteries.

Example 12: Li—Se Rechargeable Battery Assembly and Testing

The cathode discs from Example 11 were used to assemble Li—Se rechargeable coil cell batteries in the manner described in the example discussed next and shown in FIG. 15. In this example, a 10 mm diameter cathode disc 4 from Example 11 was placed onto a base 2 of a 2032 stainless steel coin cell can which functions as the positive case of the coin cell (the “positive case” in FIG. 15) with the immobilized selenium sheet 5 facing upward, away from base 2 of the positive case and with the bare Al side facing and in-contact with the base 2 of the positive case. Next, a battery separator 6 (19 mm in diameter and 25 microns in thickness) was placed on top of the cathode disc 4 in contact with the immobilized selenium sheet 5. In an example, the battery separator 6 can be an organic separator, or an inorganic separator, or a solid electrolyte separator. The organic separator can be a polymer, for example, polyethylene, polypropylene, polyester, a halogenated polymer, a polyether, a polyketone, etc. The inorganic separator can be made from glass and/or quartz fiber.

Next, 240 μL of electrolyte 7 comprising LiPF₆ (1M) in ethylene carbonate (EC) and dimethyl carbonate (DMC) solvent (50-50 in weight) was introduced into the positive case 2 followed by placing a lithium foil disc 8 (15.6 mm in diameter and 250 microns in thickness) on a side of the separator 6 opposite the cathode disc 4. Next, a stainless steel (SS) spacer 10 was placed on a side of the lithium foil disc 8 opposite the separator 6 followed by placing one or more foam discs 12 made from, for example, nickel on a side of the SS spacer 10 opposite the lithium foil disc 8. The lithium foil 8, the SS spacer 10, and/or foam 12 disk can function as an anode. Finally, a case 14 made from 2032 stainless steel 14, to function as the negative of the coin cell (the “negative case” in FIG. 15), was placed on a side of the nickel foam disk(s) 12 opposite the SS spacer 10 and on the rim of the positive case 2. The positive case 2 and the negative case 14 were then sealed together under high pressure, e.g., 1,000 psi. The sealing of the positive and negative cases (2, 14) under high pressure also had the effect of compressing together the stack comprising (from bottom to top in FIG. 15) the cathode disc 4, separator 6, lithium foil 8, SS spacer 10, and Ni foam disc(s) 12. More than a dozen coin cell batteries were assembled using the battery separators described above and fiberglass separators. The assembled coil cell batteries were then tested under following conditions.

Some of the assembled coin cell batteries were tested under charge-discharge rates of 0.1 C and 1 C by using a Lanhe Battery Tester CT2001A. Each coin cell battery was tested: (1) rest for 1 hour; (2) discharge to 1V; (3) rest for 10 minutes; (4) charge to 3V; (5) rest for 10 minutes; repeat steps (2) to (5) for repeating cycling test.

FIG. 16A (left) shows the cycling test results (313 cycles at a charge-discharge rate of 0.1 C) for a coin cell made in accordance with Example 12 using the cathode prepared in accordance with Example 11, showing excellent cycling stability, having a specific capacity of 633.7 mAh/g after 313 cycles, which is a 93.4% retention of initial specific capacity. The first discharge specific capacity was higher than stoichiometric value, possibly due to some side reactions on the cathode and anode surfaces. From the second cycle on, the specific capacity decreased with cycling initially; however, the specific capacity increased slightly from about 30 cycles to about 120 cycles before staying stable to about 180 cycles and then decreasing. FIG. 16B (right) also shows excellent cycling stability (100 cycles at 0.1 C and then 500 cycles at 1 C) for another coin cell, having a specific capacity of 462.5 mAh/g at the 600^(th) cycle, which is a 66.0% retention of the 2^(nd) cycle capacity at 0.1 C or a 80.3% retention of the 105^(th) cycle capacity at 1 C. The Coulombic efficiency can be ≥95%, ≥98%, or as high as 100%, suggesting that there was no detectable amount of selenium being shuttled between cathode and anode. This electrochemical performance is believed to be the results of the immobilized selenium in the cathode, preventing selenium from being dissolved and shuttled from cathode 14 to anode 2.

FIG. 17 shows cycling test results at different discharge-charge cycling rates (between 0.1 C and 10 C-rate) for coin cell that was assembled with a polymer separator described in Example 12. The testing protocols were similar to the tests described above except for the cycling rates (0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C); five cycles of charge and discharge were performed for each C-Rate; then the cycling rate was returned to 0.1 C cycle. At the 0.1 C rate, the battery exhibited a specific capacity around stoichiometric value. In addition, the battery exhibited good stability in cycling for cycling rates of 0.2 C, 0.5 C, 1 C, and 2 C. The battery also exhibited fast charging and discharging capability, cycled 56% of stoichiometric capacity at 10 C-rate, though showing a declining specific capacity along cycling. In other words at the 10 C-rate the battery took 3.3 minutes to charge and discharge to/from a capacity of 56% of the stoichiometric value. Under such fast cycling rate, a conventional battery would not be expected to survive.

The Li—Se battery comprising immobilized selenium can recover its specific capacity to 670 mAh/g, 98% of its full capacity when cycled at 0.1 C-rate at the beginning of the test. It is believed that (1) the stabilization of selenium in the immobilized selenium cathode avoids selenium from leaving the carbon skeleton, avoiding the selenium from being shuttled between the cathode and anode during cycling, which enables the battery to have improved cycling performance; (2) both Sp² carbon-carbon bonds and carbon skeleton, selenium-selenium bonds, and carbon-selenium interactions may be under compression, possibly resulting in superior electrical conductivity within the carbon skeleton, within selenium particles, and among carbon and selenium interfaces, which may aid in achieving the observed cycling performance at high C-rates.

The immobilized selenium body comprising selenium and carbon prepared in accordance with the principles described herein can comprise one or more of the following features:

(a) a kinetic energy required for a selenium particle to escape the immobilized selenium can be ≥9.5 U/mole, ≥9.7 kJ/mole, ≥9.9 kJ/mole, ≥10.1 U/mole, ≥10.3 U/mole, or ≥10.5 kJ/mole;

(b) a temperature required for a selenium particle to escape the immobilized selenium can be can be ≥490° C., ≥500° C., ≥510° C., ≥520° C., ≥530° C., ≥540° C., ≥550° C., or ≥560° C.;

(c) the carbon can have a surface area (for pores less than 20 angstroms) ≥500 m²/g, ≥600 m²/g, ≥700 m²/g, ≥800 m²/g, ≥900 m²/g, or ≥1,000 m²/g;

(d) the carbon can have a surface area (for pores between 20 angstroms and 1000 angstroms) ≤20%, ≤15%, ≤10%, ≤5%, ≤3%, ≤2%, ≤1% of the total surface area;

(e) the carbon and/or selenium can be under compression. Benefits of the immobilized selenium where the carbon and/or selenium are under compression versus a carbon-selenium system where the carbon and/or selenium are not under compression can include: improved electron flow, reduced resistance to electron flow, or both, which can facilitate electron delivery to the selenium and from selenium anions during charging and discharging of a rechargeable battery that has a cathode comprised of the immobilized selenium;

(f) the immobilized selenium can comprise selenium that has an activation energy higher than the activation energy higher for conventional (non-immobilized) selenium in order for the selenium to escape from the immobilized Se—C composite system. In an example, the activation energy for non-immobilized selenium (Se-Super P composite system) was determined to be 92 kJ/mole, according to ASTM Method E1641-16. In contrast, in an example, the activation energy for selenium in the immobilized selenium comprising selenium and carbon can be ≥95 U/mole, ≥98 U/mole, ≥101 U/mole, ≥104 kJ/mole, ≥107 kJ/mole, or 110 kJ/mole. In another example, the activation energy for selenium in the immobilized selenium comprising selenium and carbon can be ≥3%, ≥6%, ≥9%, ≥12%, ≥15%, or ≥18% greater than that for selenium in Se-Super P composite;

(g) the immobilized selenium can comprise selenium that has higher collision frequency than non-immobilized selenium. In an example, the collision frequency for non-immobilized selenium was determined to be 2.27×10⁵, according to the ATSM Method E1641-16. In contrast, in an example, the collision frequency for selenium in immobilized selenium, comprising selenium and carbon, can be is ≥2.5×10⁵, ≥3.0×10⁵, ≥3.5×10⁵, ≥4.0×10⁵, ≥4.5×10⁵, ≥5.0×10⁵, ≥5.5×10⁵, ≥6.0×10⁵, or ≥8.0×10⁵. The immobilized selenium can have a collision frequency ≥10%, 30%, ≥50%, ≥80%, 100%, ≥130%, ≥150%, ≥180%, or ≥200% than for non-immobilized selenium in an Se—C composite; and

(h) the immobilized selenium can comprise selenium that has a kinetic rate constant that is ≤⅕, ≤ 1/10, ≤ 1/50, ≤ 1/100, ≤ 1/500, or ≤ 1/1000 of the kinetic rate constant for non-immobilized/conventional selenium. In an example, the immobilized selenium can comprise selenium that has a kinetic rate constant (at 50° C.) of ≤1×10⁻¹⁰, ≤5×10⁻¹¹, ≤1×10⁻¹¹, ≤5×10⁻¹², or ≤5×10⁻¹³.

With the carbon and/or selenium of the immobilized selenium under compression, the D-band and/or the G-band of Raman spectrum for the Sp² C—C bonds of the carbon (or carbon skeleton defined by said carbon) of the immobilized selenium can show a red (positive) shift, e.g., by ≥1 cm⁻¹, ≥2 cm⁻¹, ≥3 cm⁻¹, ≥4 cm⁻¹, or ≥5 cm⁻¹ from a carbon feedstock.

With the carbon and/or selenium of the immobilized selenium under compression, the selenium can have a red (positive) shift from the Raman peak of pure selenium (235 cm⁻¹), e.g., by ≥4 cm⁻¹, ≥6 cm⁻¹, ≥8 cm⁻¹, ≥10 cm⁻¹, ≥12 cm⁻¹, ≥14 cm⁻¹, or ≥16 cm⁻¹, which red shift can suggest compression on the selenium particles.

The immobilized selenium can be an elemental form of selenium and/or a compound form selenium.

The immobilized selenium comprising selenium and carbon can be also doped with one or more additional element(s) from Group 6 of the Periodic Table (hereinafter, “additional G6 element(s)”), including, for example, without limitation, sulfur and/or tellurium. The dopant level may range from as low as 100 ppm by weight to as high as 85% of the total weight of the immobilized selenium. In an example, the immobilized selenium can comprise 15%-70% carbon and 30%-85% selenium and, optionally, additional G6 element(s). In an example, the immobilized selenium can comprise (1) 15%-70% carbon and (2) 30%-85% selenium+additional G6 element(s) mixture. In the mixture comprising selenium+additional G6 element(s), the additional G6 element(s) can comprise between 0.1%-99% of the mixture and selenium can comprise between 1%-99.9% of the mixture. However, these ranges of selenium+additional G6 element(s) are not to be construed in a limiting sense.

The immobilized selenium can include ≥5% selenium, ≥10% selenium, ≥20% selenium, ≥30%, ≥40% selenium, ≥50% selenium, ≥60% selenium, or ≥70% or higher selenium.

The immobilized selenium can optionally including another element, such as, for example, sulfur, tellurium, etc.

The immobilized selenium can be Raman-inactive or Raman-active. If Raman-active, the immobilized selenium can have a Raman relative peak intensity at 255±25 cm⁻¹, at 255±15 cm⁻¹ or at 255±10 cm⁻¹.

The immobilized selenium can comprise selenium having a Raman relative peak intensity of ≥0.1%, ≥0.5%, ≥1%, ≥3%, or ≥5%, herein, the Raman relative peak intensity is defined as the area of the Raman peak at 255 cm⁻¹ relative to the area of the D-band peak of the carbon Raman spectrum.

The carbon comprising the immobilized selenium can serve as a carbon skeleton for selenium immobilization. The carbon skeleton can have Sp²-carbon-carbon bonds with a Raman D-band located at 1365±100 cm⁻¹ and G-band located at 1589±100 cm⁻¹; a D-band located at 1365±70 cm⁻¹ and a G-band located at 1589±70 cm⁻¹; a D-band located at 1365±50 cm⁻¹ and a G-band located at 1589±50 cm⁻¹; a D-band located at 1365±30 cm⁻¹ and a G-band located at 1589±30 cm⁻¹; or a D-band located at 1365±20 cm⁻¹ and a G-band located at 1589±20 cm⁻¹.

The carbon of the immobilized selenium can include Sp² carbon-carbon bonds, having Raman peaks featuring a D-band and a G-band. A ratio of the area of D-band to G-band can range from 0.01 to 100, from 0.1 to 50, or from 0.2 and 20.

The carbon of the immobilized selenium can include Sp² carbon-carbon bonds, having Raman peaks featuring a D-band and a G-band. Each of the D-band and the G-band can have a shift to a higher wavenumber ≥1 cm⁻¹, ≥2 cm⁻¹, or more.

The carbon of the immobilized selenium can be doped with one or more other elements in the period table.

The carbon of the immobilized selenium can be porous. The pore size distributions of the carbon skeleton can range between one angstrom to a few microns. The pore size distribution can have at least one peak located between one angstrom and 1000 angstroms, between one angstrom and 100 angstroms, between one angstrom and 50 angstroms, between one angstrom and 30 angstroms, or between one angstrom and 20 angstroms. The porosity of the carbon skeleton can have pore size distributions with more than one peak in the foregoing ranges.

The carbon of the immobilized selenium can include a pore volume between 0.01 mL/g and 5 mL/g; between 0.01 mL/g and 3 mL/g; between 0.03 mL/g and 2.5 mL/g; or between 0.05 mL/g and 2.0 mL/g.

The carbon of the immobilized selenium can include a pore volume (that has pore size <100 angstroms, <50 angstroms, <30 angstroms, or <20 angstroms) that can be >30%, >40%, >50%, >60%, >70%, or >80% of the total measurable pore volume.

The carbon of the immobilized selenium can include a surface area >400 m²/g, >500 m²/g, >600 m²/g, >700 m²/g, >800 m²/g, >900 m²/g, or >1000 m²/g.

The carbon of the immobilized selenium can be amorphous and can have a broad peak centered at a d-spacing around 5.2 angstroms.

The carbon of the immobilized selenium can be of any morphology, platelet, sphere, fiber, needle, tubular, irregular, interconnected, agglomerated, discrete, or any solid particles. Platelet, fiber, needle, tubular, or some morphology having a certain level of aspect ratio may be beneficial for achieving better inter-particle contact, resulting in enhanced electrical conductivity (over immobilized selenium made from a different aspect ratio), which may be beneficial to an electrochemical cell, such as a rechargeable battery.

The carbon of the immobilized selenium can be of any particle size, having a median particle size between 1-9 nanometers and 2 millimeters, between 1-9 nanometers to <1000 microns, or between 20 nanometers to 100 microns.

The selenium of the immobilized selenium can be amorphous, e.g., as determined by X-ray diffraction. The diffraction peak of the selenium of the immobilized selenium, which can have a d-spacing about 5.2 angstroms may be weaker than the diffraction peak that for the carbon skeleton, e.g., 10% weaker, 20% weaker, 30% weaker, or 40% weaker.

In an example, a method of preparing the immobilized selenium can include:

(a) physical mixing carbon and selenium. The physical mixing can be by ball-milling (dry and wet), mixing with mortar and pestle (dry or wet), jet-milling, horizontal milling, attrition milling, high shear mixing in slurries, regular slurry mixing with blade, etc.;

(b) the physically mixed carbon and selenium of step (a) can be heated at the melting temperature of selenium or higher. The heating of the carbon and selenium mixture can occur in the presence of an inert gas environment such as, but not limited to, argon, helium, nitrogen, etc., or in an air or reactive environment;

(c) optionally homogenizing or blending the heated carbon and selenium to achieve selenium immobilization; and

(d) cooling the immobilized selenium of step (c) to ambient or room temperature.

In another example, immobilized selenium can be prepared by dissolving selenium onto carbon followed by evaporation. The solvent for dissolving the selenium can be an alcohol, an ether, an ester, a ketone, a hydrocarbon, a halogenated hydrocarbon, a nitrogen-containing compound, a phosphorus containing compound, a sulfur-containing compound, water, etc.

In another example, the immobilized selenium can be prepared by melting selenium onto carbon, followed by removing extra or excess non-immobilized selenium.

In an example, a method of making the immobilized selenium can include:

(a) mixing selenium and carbon together under dry or wet conditions;

(b) optional drying the mixture of step (a) at an elevated temperature;

(c) optional pelletizing the dried mixture of step (b);

(d) melting the selenium into the carbon to produce the immobilized selenium.

Immobilized selenium can be used as a cathode material for a rechargeable battery. The cathode can include an inorganic or an organic binder. The inorganic binder can be a natural product, such as, for example, CMC, or a synthetic product, such as, for example, SBR Rubber latex. The cathode can include an optional electric-conductivity promoter, such as, for example, graphite-derived small particles, graphene, carbon nano-tubes, carbon nano-sheet, carbon blacks, etc. Finally, the cathode can include a charge collector such as, for example, an aluminum foil, a copper foil, a carbon foil, a carbon fabric, or other metallic foil.

The method of making the cathode can include coating an immobilized selenium-containing slurry onto the charge collector, followed drying the slurry coated charge collector (e.g., air dry, oven-dry, vacuum oven-dry, etc.). The immobilized selenium can be dispersed into the slurry, which can be prepared by a high shear mixer, a regular mixer, a planetary mixer, a double-planetary mixer, a ball mill, a vertical attritor, a horizontal mill, etc. The slurry can then be coated onto the charge collector, followed by drying in air or in vacuum. The coated cathode can then be pressed or roller-milled (or calendared) prior to its use in a rechargeable battery.

A rechargeable battery can be made using the immobilized selenium described herein. The rechargeable battery can include a cathode comprising the immobilized selenium, an anode, and a separator separating the anode and the cathode. The anode, the cathode, and the separator can be immersed in an electrolyte, such as, for example, LiPF₆. The anode can be comprised of lithium, sodium, silicon, graphite, magnesium, tin, etc.

The separator can be comprised of an organic separator, an inorganic separator, or a solid electrolyte separator. The organic separator can comprise a polymer such as, for example, polyethylene, polypropylene, polyester, a halogenated polymer, a polyether, a polyketone, etc. The inorganic separator can comprise a glass or quartz fiber, or a solid electrolyte separator.

The electrolyte can comprise a lithium salt, a sodium salt, or other salt from Group IA, IIA, and IIA, in an organic solvent. The organic solvent can comprise an organic carbonate compound, an ether, an alcohol, an ester, a hydrocarbon, a halogenated hydrocarbon, a lithium containing-solvent, etc.

The rechargeable battery can be used for electronics, an electric or hybrid vehicle, an industrial application, a military application, such as a drone, an aerospace application, a marine application, etc.

The rechargeable battery can have an electrochemical capacity of ≥400 mAh/g active amount of selenium, ≥450 mAh/g active amount of selenium, ≥500 mAh/g active amount of selenium, ≥550 mAh/g active amount of selenium, or ≥600 mAh/g active amount of selenium.

The rechargeable battery can undergo electrochemical cycling for ≥50 cycles, ≥75 cycles, ≥100 cycles, ≥200 cycles, etc.

The rechargeable battery can be charged and/or discharged at 0.1 C, 0.2 C, 0.5 C, 1 C, 1.5 C, 2 C, 3 C, 4 C, 5 C, 6 C, 7 C, 8 C, 9 C, 1° C. or faster.

The rechargeable battery can retain a battery specific capacity >30%, >40%, >50%, >60%, >70%, or >80% of the 2^(nd) discharge specific capacity at a cycling rate of 0.1 C after conducting high C-Rate charge-discharge cycling (5 cycles at 0.1 C, 5 cycles at 0.2 C, 5 cycles at 0.5 C, 5 cycles at 1 C, 5 cycles at 2 C, 5 cycles at 5 C, and 5 cycles at 1° C.).

The rechargeable battery can have a Coulombic efficiency ≥50%, ≥60%, ≥70%, ≥80%, ≥90%, or as high as around 100%.

Coloumbic efficiency of a battery is defined as follows:

$\eta_{C} = \frac{Q_{out}}{Q_{in}}$

Where η_(c) is the Coloumbic efficiency (%) Q_(out) is the amount of charge that exits the battery during a discharge cycle. Q_(in) is the amount of charge that enters the battery during a charging cycle.

The rechargeable battery can be charged at C-rate of 0.1 C, 0.2 C, 0.5 C, 1 C, 1.5 C, 2 C, 3 C, 4 C, 5 C, 6 C, 7 C, 8 C, 9 C, 1° C. or faster. A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. For example, a 1 C rate means that the discharge current will discharge the entire battery in 1 hour. For example, for a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps. A 5 C rate for this same battery would be 500 Amps, and a 0.5 C rate would be 50 Amps.

The cathode of the rechargeable battery can comprise one or more elements of a chalcogen group such as selenium, sulfur, tellurium, and oxygen.

The anode of the rechargeable battery can comprise at least one element of alkali metal, alkali earth metals, and group IIA metals.

The separator of the rechargeable battery can comprise an organic separator or an inorganic separator.

The electrolyte of the rechargeable battery can comprise at least one element of alkali metals, alkali earth metals, and Group IIIA metals; and a solvent of the electrolyte can comprise an organic solvent, carbonate-based, ether-based, or ester-based.

The rechargeable battery can have a specific capacity of ≥400 mAh/g, ≥450 mAh/g, ≥500 mAh/g, ≥550 mAh/g, or ≥600 mAh/g.

The rechargeable battery can undergo electrochemical cycling for ≥50 cycles, ≥75 cycles, ≥100 cycles, ≥200 cycles, etc.

The rechargeable battery can have a specific capacity >30%, >40%, >50%, >60%, >70%, or >80% of the 2^(nd) discharge specific capacity at a cycling rate of 0.1 C after conducting high C-Rate charge-discharge cycling (5 cycles at 0.1 C, 5 cycles at 0.2 C, 5 cycles at 0.5 C, 5 cycles at 1 C, 5 cycles at 2 C, 5 cycles at 5 C, and 5 cycles at 10 C).

The rechargeable battery can have has a Coulombic efficiency ≥50%, ≥60%, ≥70%, ≥80%, or ≥90%.

Also disclosed is a composite comprising selenium and carbon, said composite can have a platelet morphology with an aspect ratio of ≥1, ≥2, ≥5, ≥10, or ≥20.

The selenium of the composite can be amorphous, e.g., as determined by X-ray diffraction. The diffraction peak of the selenium can have a d-spacing about 5.2 angstroms which may be weaker than that for a carbon skeleton, e.g., 10% weaker, 20% weaker, 30% weaker, or 40% weaker than the carbon skeleton.

In an example, the method of preparing the composite can include:

(a) physical mixing carbon and selenium. The physical mixing can be by ball-milling (dry and wet), mixing with mortar and pestle (dry or wet), jet-milling, horizontal milling, attrition milling, high shear mixing in slurries, regular slurry mixing with blade, etc.;

(b) the physically mixed carbon and selenium of step (a) can be heated to the melting temperature of selenium or higher and said heating can occur in the presence of an inert gas environment such as, for example, argon, helium, nitrogen, etc., or in an air or reactive environment; and

(c) the heated carbon and selenium of step (b) can be homogenized or blended as an aid to achieving selenium immobilization.

In another example, the composite can be prepared by dissolving selenium onto carbon followed by evaporation. The solvent for dissolving the selenium can include an alcohol, an ether, an ester, a ketone, a hydrocarbon, a halogenated hydrocarbon, a nitrogen-containing compound, a phosphorus containing compound, a sulfur-containing compound, water, etc.

The composite can be prepared by melting selenium onto (or into) carbon, followed by removing extra or excess non-immobilized selenium.

In an example, a method of making the composite can include:

(a) mixing selenium and carbon together under dry or wet conditions;

(b) optional drying the mixture of step (a) at an elevated temperature;

(c) optional pelletizing the dried mixture of step (b);

(d) melting the selenium into the carbon to produce the immobilized selenium.

The composite can be used as a cathode material for a cathode of a rechargeable battery. The cathode can include an inorganic or an organic binder. The inorganic binder can be a natural product, such as, for example, CMC, or a synthetic product, such as, for example, SBR Rubber latex. The cathode can include an optional electric-conductivity promoter, such as, for example, graphite-derived small particles, graphene, carbon nano-tubes, carbon nano-sheet, carbon blacks, etc. Finally, the cathode can include an electric charge collector such as, for example, an aluminum foil, a copper foil, a carbon foil, a carbon fabric, or other metallic foil.

The method of making the cathode can include coating an immobilized selenium-containing slurry onto the charge collector, followed by drying the slurry coated charge collector (e.g., air dry, oven-dry, vacuum oven-dry, etc.). The immobilized selenium can be dispersed into the slurry, which can be prepared by a high shear mixer, a regular mixer, a planetary mixer, a double-planetary mixer, a ball mill, a vertical attritor, a horizontal mill, etc. The slurry can then be coated onto the charge collector, followed by drying in room air or in a vacuum. The coated cathode can then be pressed or roller-milled (or calendared) prior to its use in a rechargeable battery.

A rechargeable battery can be made using the above-described composite. The rechargeable battery can be charged at 0.1 C, 0.2 C, 0.5 C, 1 C, 1.5 C, 2 C, 3 C, 4 C, 5 C, 6 C, 7 C, 8 C, 9 C, 10 C or faster.

Example 13: Preparation of Immobilized Selenium Doped with Sulfur, Electrode, and Batteries Thereof

Following the principles and procedures described in Example 10, 5 atomic percent (at %) of selenium, 20 at % of selenium, 35 at % of selenium, and 50 at % of selenium were separately replaced by sulfur in the synthesis of immobilized sulfur-doped selenium detailed in the following Table 4. Samples of the sulfur-doped immobilized selenium were synthesized with the carbon skeleton prepared in accordance with the principles and procedures described Example 9.

TABLE 4 Sample ID Se, at % S, at % Se, wt % S, wt % Se95S5 95 5 97.9 2.1 Se80S20 80 20 90.8 9.2 Se65S35 65 35 82.1 17.9 Se50S50 50 50 71.1 28.9

The thus prepared samples of immobilized sulfur-doped selenium were then used to prepare a number of cathodes 4 comprising immobilized sulfur-doped selenium in accordance with the principles and procedures described in Example 11 for immobilized selenium.

The thus prepared cathodes comprising immobilized sulfur-doped selenium in this example were then used to prepare coin cell batteries in accordance with the principles and procedures described in Example 12.

The assembled coin cell batteries in this example were then tested in the battery tester described in Example 12, following the same testing protocols also described in Example 12, at 0.1 C and 1 C charging and discharging cycling rates.

The electrochemical cycling results at 0.1 C for coil cell batteries including a cathode comprised of immobilized sulfur-doped selenium cathode made with immobilized sulfur-doped selenium sample (Se50S50 in Table 4) are shown in FIG. 18, having a 2^(nd) cycle discharge capacity of 821 mAh/g (which is considered good) and a steady Coulombic efficiency ≥95%, typically ≥98% (which is also considered good), or as high as 100%.

If selenium is assumed to have a stoichiometric specific capacity of 675 mAh/g at the 0.1 C cycling rate, then sulfur specific capacity would be estimated to be about 1,178 mAh/g (which is considered good for sulfur). The Coulombic efficiency ≥95%, ≥98%, or as high as 100% indicates that there is no significant amount of sulfur being shuttled between the cathode and anode. Sulfur species in the immobilized sulfur-doped selenium battery function well in an electrolyte comprising carbonate. Typically, sulfur would not be expected to function well in a Li—S battery having carbonate as the electrolyte; a conventional Li—S battery typically uses an ether-based electrolyte. Carbonate-based electrolyte is typically used in present lithium-ion batteries. Carbonate-based electrolyte is more economical and much more widely available in the market place, as compared to ether-based electrolyte.

The electrochemical cycling results at the 1 C cycling rate for coil cell batteries including a cathode comprised of immobilized sulfur-doped selenium cathode made with immobilized sulfur-doped selenium sample (Se50S50 in Table 4) are shown in FIG. 19, having a 2^(nd) cycle discharge capacity of 724 mAh/g and a steady Coulombic efficiency ≥95%, typically ≥98%, or as high as 100%.

If selenium is assumed to have a specific capacity of 625 mAh/g at the 1 C cycling rate, then the sulfur specific capacity would be estimated to be about 966 mAh/g (which is also unexpected). Sulfur is an insulator and has a very low electrical conductivity. Typically, a Li—S battery cannot cycle well at a fast cycling rate, such as at 1 C rate.

As can be seen, when used as a cathode material in a rechargeable battery, immobilized sulfur-doped selenium overcomes two fundamental issues associated with Li—S batteries, namely, shuttling effect and low cycling rate. With these two issues resolved, a battery including a cathode comprised of immobilized sulfur-doped selenium can have high energy density and high power density in real applications.

As can been seen, in an example, an immobilized sulfur-doped selenium system or body can be formed by the method comprising: (a) mixing selenium, carbon, and sulfur to form a selenium-carbon-sulfur mixture; (b) heating the mixture of step (a) to a temperature above the melting temperature of selenium; and (c) causing the heated mixture of step (b) to cool to ambient or room temperature, thereby forming the immobilized sulfur-doped selenium body.

The immobilized sulfur-doped selenium body of step (c) can comprise selenium and sulfur in a carbon skeleton body.

Step (a) can occur under a dry or a wet condition.

Step (b) can include homogenizing or blending the mixture.

Step (a) can include forming the selenium-carbon-sulfur mixture into a body. Step

(b) can include heating the body to a temperature above the melting temperature of selenium. Step (c) can include causing or allowing the body to cool to ambient or room temperature.

Step (b) can include heating the mixture for a sufficient time for the selenium and carbon and sulfur to fully or partially react.

In another example, a method of preparing an immobilized sulfur-doped selenium system or body can comprise: (a) forming a carbon skeleton; and (b) melting selenium and sulfur into the carbon skeleton.

In another example, a method of forming an immobilized sulfur-doped selenium system or body can comprise: (a) mixing selenium and carbon and sulfur; and (b) following step (a), causing the selenium and sulfur to dissolve onto the carbon thereby forming the immobilized sulfur-doped selenium system or body.

A solvent for dissolving the selenium and sulfur can be an alcohol, an ether, an ester, a ketone, a hydrocarbon, a halogenated hydrocarbon, a nitrogen-containing compound, a phosphorus containing compound, a sulfur-containing compound, or water. The solvent can be added to one or more of the selenium, the sulfur, or the carbon prior to step (a), during step (a), or during step (b).

The method can further including (c) removing excess non-immobilized selenium, non-immobilized sulfur, or both from the immobilized sulfur-doped selenium system or body.

Also disclosed is a rechargeable battery comprising: a cathode comprised of immobilized sulfur-doped selenium disposed on an electrically conductive substrate; a separator disposed in direct contact with the electrically conductive substrate, and in contact with the immobilized sulfur-doped selenium; and an anode spaced from the cathode by the separator.

The rechargeable battery can further include the anode spaced from the separator by lithium. In an example, the lithium can be in the form of a lithium foil.

The rechargeable battery can further include the cathode, the separator, the anode, and the lithium immersed in an electrolyte.

In the rechargeable battery the immobilized sulfur-doped selenium can comprise a selenium-carbon-sulfur mixture, wherein the selenium and sulfur has been melted into the carbon.

In the rechargeable battery the separator can be formed from an organic material, an inorganic material, or a solid electrolyte.

The rechargeable battery can have a Coulombic efficiency ≥95%.

Having thus described a method of preparing a selenium carbon composite material, a method of making immobilized selenium and the use of the immobilized selenium, e.g., in a rechargeable battery, a method of making immobilized selenium in a porous carbon with the presence of a desirable level of oxygen species and the use of the immobilized selenium in a rechargeable battery will be described.

Please note that selenium is one of chalcogens which comprise sulfur, selenium, tellurium, etc. The descriptions related to selenium in the present disclosure are equally applicable to the rest of chalcogen elements such as sulfur and tellurium.

In the present invention, selenium atoms or molecules are immobilized, either in their oxidized form as an element (in a charged state in a rechargeable battery) or in their reduced form as selenide (in a discharged state in a rechargeable battery), selenium atoms are localized inside the pores of a porous carbon, allowing electrochemical reactions of a rechargeable battery to cycle properly during discharge process and during charge process. During discharge process, an elemental selenium atom (neutral, Se) at the cathode is reduced and gains two electrons to become a selenide anion (Se²⁻), typically existing as a salt, lithium selenide, Li₂Se, and still localized at the cathode. The chemical energy of the discharge process is effectively converted to electrical energy of the rechargeable battery, with a minimum level of conversion to heat due to the presence of the intrinsic internal electrical resistance of the battery. During charge process of a chargeable battery, electrical energy is also effectively converted into chemical energy due to which a selenide anion in Li₂Se form at the cathode is oxidized and loses two electrons to form an elemental selenium atom that is still localized at the cathode. If selenium is not immobilized in a porous carbon, intermediate selenium species (typically in a polyselenide form, Se_(n) ²⁻) may form in a cathode during the electrochemical reduction/oxidation (ReDox) process and dissolve in a liquid electrolyte of the battery system. The dissolved intermediate polyselenide species is then transported in the liquid electrolyte from the cathode through a battery separator to anode where it finds metallic lithium, reacts with metallic lithium atoms, and forms a salt consisting of lithium and selenium on the surface of the lithium anode. The chemical energy generated from the reactions of lithium and intermediate selenium species on the anode is converted into heat, instead of electrical energy, which is highly undesirable. In addition, the selenium salt formed on the anode may then be converted back to a polyselenide anion, even partially, which is then dissolved in the electrolyte of the battery, and transported back to the cathode where it consumes additional electrical energy by gaining electrons and gets reduced back to elemental selenium. Conversion of electrochemical energy of selenium into heat or consuming additional electrical energy is undesirable, even if the rechargeable battery can be cycled, due to an undesirably low cycling efficiency. Please note that a battery with an undesirably low cycling efficiency often fails to function properly for a desirable number of cycles.

Therefore, the present invention embodies that immobilization of selenium in a porous carbon skeleton is desirable to achieve proper cycling of a rechargeable battery comprising lithium anode, selenium cathode, a separator, and an electrolyte. Selenium is immobilized in a porous carbon and has an activation energy that, in an example, can be ≥95 kJ/mole, ≥98 kJ/mole, ≥101 kJ/mole, ≥104 kJ/mole, ≥107 kJ/mole, or ≥110 kJ/mole. The interaction of selenium and carbon in the immobilized selenium comprising selenium and porous carbon is often more frequent, having a collision frequency, in an example, ≥2.5×10⁵, ≥3.0×10⁵, ≥3.5×10⁵, ≥4.0×10⁵, ≥4.5×10⁵, ≥5.0×10⁵, ≥5.5×10⁵, ≥6.0×10⁵, or ≥8.0×10⁵. The kinetic rate constant (at 50° C.) for the immobilized selenium comprising selenium and porous carbon is, in an example, ≤1×10⁻¹⁰, ≤5×10⁻¹¹, ≤1×10⁻¹², ≤5×10⁻¹², or ≤5×10⁻¹³. The carbon skeleton plays an important role in proper immobilization of selenium. Carbon having certain amount of microporosity (pore size of 20 angstroms or smaller) is desirable to spatially confine selenium inside the micropores of carbon skeleton in order to achieve immobilization of selenium with desirable levels of activation energy, collision frequency, and kinetic rate constant. The presence of certain amounts of mesoporosity (pore size between 20 angstroms and 500 angstroms) and/or macroporosity (pore size greater than 500 angstroms) in the carbon skeleton is also desirable for successful transport of lithium ions into cathode during battery discharge process and out of cathode during battery charge process, though not critical for immobilization of selenium. The amount of microporosity, mesoporosity, or macroporosity is typically characterized by the amount of pore volume (mL) per weight unit (g) of the carbon skeleton. High level amounts of porosity including microporosity, mesoporosity, and macroporosity may be needed to spatially confine a high level of selenium in the porous carbon. The more the selenium is loaded into the porous carbon skeleton (per gram based), the more chemical energy and electrical energy can be interconverted during the cycling processes of a rechargeable battery.

The amounts of microporosity for the porous carbon for selenium immobilization may be ≥0.3 mL/g, ≥0.4 mL/g, or 20.5 mL/g. The total amounts of porosity including microporosity, mesoporosity, and macroporosity are ≥0.4 mL/g, ≥0.5 mL/g, and ≥0.6 mL/g. The percentage of microporosity among the total porosity including microporosity, mesoporosity, and microposority is between 50% and 99%, 55% and 97%, and 60% and 95%.

A short path is preferred for the lithium transport between the bulk of the electrolyte and the location where selenium is immobilized in the micropores, while the presence of mesoporosity and macroporosity may be important for successful transports of lithium ions in and out to access selenium that is immobilized in micropores, allowing the electrochemical processes to function properly during discharge and charge processes of a rechargeable battery. One of the particle dimensions of the porous carbon may prefer to be small, possibly ≤5 μm, ≤2 μm, 51 μm, ≤0.5 μm, or ≤0.2 μm; the carbon particle may be a relatively large in size with interconnected thin walls, or may be small with regard to the particle as a whole. The dimension of the porous carbon may be characterized by a Scanning Electron Microscope, a transmission electron microscope, an optical microscope, a laser-scattering particle size analyzer, etc.

Selenium is immobilized in the space of the micropores of porous carbon particle that has porosity including microporosity, mesoporosity, and macroporosity. Selenium is immobilized by strong interactions between selenium and carbon skeleton inside the pores of the porous carbon. Porous carbon with a high surface area may have more active sites where selenium interacts with carbon skeleton. The porous carbon prefers to have a BET surface area ≥600 m²/g, ≥800 m²/g, ≥1,000 m²/g, ≥1,200 m²/g, or ≥1,400 m²/g.

Selenium is effectively immobilized in porous carbon through chemical interactions between the selenium species and the surface of the carbon skeleton. The oxygen-related functional groups in the porous carbon may be the key species that assert strong chemical interactions with selenium. Such strong chemical interactions may result in an effective immobilization of selenium, resulting in an increase in activation energy, a decrease in kinetic rate constant, and/or an increase in collision frequency. To achieve desirable level of chemical interactions among skeleton carbon atoms and selenium atoms or molecules the amounts of the oxygen-related groups or species, as interfaces between carbon and selenium, in the porous carbon may need to be significantly high enough so that the immobilization of selenium is effective for a rechargeable battery that can function properly in the electrochemical discharge and charge cycling processes. The amounts of oxygen-related species, or oxygen functional group, may be characterized by the amounts of oxygen contents in the porous carbon, analyzed by an Oxygen Analyzer (such as LECO oxygen analyzer). The oxygen content of the porous carbon may also be characterized by other instruments that function similarly to LECO instruments, for example, temperature-programmed desorption (or thermal gravimetric analysis, or TGA) with a gas detector such as, but not limited to, a mass-spec detector, a thermal conductivity detector, etc., with an option of a comprehensive cold trap mechanism.

Porous carbon for selenium immobilization prefers to have the amounts of oxygen content ≥2%, ≥3%, ≥5%, or more preferred, ≥7%.

The oxygen species in the porous carbon may be categorized and be characterized by Temperature-Programmed-Desorption (TPD), typically CO₂ forming oxygen species, CO forming oxygen species, and water forming oxygen species. In a flow of an inert gas such as helium, nitrogen, or argon as a carrier gas, the sample of a porous carbon is temperature-programmed heated at a defined heating rate to a temperature that may be as high as 1,000° C. Oxygen species in the porous carbon sample are destroyed at different temperature, forming CO₂, CO, and/or water, which elute by the carrying gas. For the porous carbon that is used for selenium immobilization, the ratio of amounts of oxygen related to CO₂ formation to the amounts of oxygen related to CO formation may be between 0.05 to 0.95, 0.15 to 0.85, or 0.2 to 0.8; oxygen amount related to CO₂ formation may be greater than ≥2%, ≥3%, ≥5%, or more preferred ≥7%; oxygen amount related to CO formation may be greater than ≥2%, ≥3%, ≥5%, or more preferred ≥7%; oxygen amount related to water formation may be greater than ≥2%, ≥3%, ≥5%, or more preferred ≥7%.

In the other aspect, the density of oxygen species (micro-mole per square meter, or μmole/m²) of skeleton carbon may be critical for achieving sufficient level of chemical interactions between carbon surface and selenium atoms, resulting in satisfactory immobilization of selenium for proper cycling of a rechargeable battery. Porous carbon for selenium immobilization prefers to have the amounts of oxygen content ≥0.8 μmole/m², ≥1.0 μmole/m², ≥1.2 μmole/m², ≥1.4 μmole/m², or more preferred, ≥1.6 μmole/m²; or oxygen amount related to CO₂ formation is ≥0.8 μmole/m², ≥1.0 μmole/m², ≥1.2 μmole/m², ≥1.4 μmole/m², or more preferred, ≥1.6 μmole/m²; or oxygen amount related to CO formation is ≥0.8 μmole/m², ≥1.0 μmole/m², ≥1.2 μmole/m², ≥1.4 μmole/m², or more preferred, ≥1.6 μmole/m².

The presence of adequate amounts of oxygen species in the skeleton carbon seems to play critical roles in oxidation stability of the Se—C composite. Under ambient conditions, the Se—C composite that is made of a carbon skeleton with low amounts of oxygen species seems to be facile to be oxidized under ambient conditions; the fresh material of such Se—C composite shows temperature-gravimetric behaviors similar to those for the Se—C composite with a carbon skeleton having adequate amounts of oxygen species; however, it was surprised to discover that Se—C composites made with low amounts of oxygen species, aged under ambient conditions for about two years and three months, show a major exothermal weight loss at temperatures around 200-250° C. with a total weight loss of the Se—C composite exceeding the amount of selenium that was loaded onto carbon when those were freshly made; in another example, the amount of the weight loss under these temperatures seems to correlate well with the amount of carbon skeleton in the Se—C composites: the higher the amounts of carbon skeleton are, the more the exothermal weight loss is at the temperatures between 200° C. and 250° C.; in another example, the oxygen contents for the aged (27 months) Se—C composites made with carbon skeleton having low amounts of oxygen species increases to as high as about 24%; in another example, it is also interesting to note that density of the aged Se—C composites that were made from the carbon skeleton having low amounts of oxygen species is reduced substantially. Without limiting the scope or spirit of the present invention, the exothermal weight loss may be related to the oxidation of carbon-selenium composite by the ambient oxygen species (environment oxygen and/or moisture under ambient conditions), and/or ambient-oxygen-species-aided oxidation of carbon by elemental selenium, both resulting in the formation of the oxidized carbon species; oxidation of the Se—C composite possibly occurs near pore mouths of the composite; the newly formed oxidized carbon species near pore mouths may form closed porosity; the closed porosity is then not accessible by the probing molecules of the helium gas during density measurements, which would result in an decrease in the density for the aged Se—C composite samples that were made with carbon skeleton having a low amounts of oxygen species. The decomposition products of oxidized carbon species at temperatures between 200° C. and 250° C. under an inert carry gas such as argon or nitrogen may be related to carbon, selenium, and/or oxygen, as an example, carbon dioxide, carbon monoxide, carbon diselenide, carbon oxyselenide, etc. Please also note that the free standing carbon skeleton with low amounts of oxygen species is stable under ambient conditions. Carbon and/or selenium in the Se—C composite that is made with a carbon skeleton having low amounts of oxygen species seem to be prone to be attacked by environmental oxygen species along with selenium. Such oxidation instability may very well translate into poor electrochemical cycling performances of a rechargeable battery of Li—Se; it is possible that carbon skeleton may be slowly oxidized by elemental selenium under electrochemical cycling environments, just like under ambient environments. It is not desirable to have formation of oxidized carbon species in a Se—C composite. The amount of selenium in the oxidized carbon species may not participate in electrochemical cycling of a rechargeable battery, resulting in a permanent loss of specific capacity of a rechargeable battery; carbon skeleton also plays key roles, besides hosting selenium for its immobilization, in providing an electrical conducting pathway for (1) delivery of electrons from the external circuit to elemental selenium during discharge process and (2) harvest of electrons from selenium anions (Se²) to the external circuit during charging process. If partial amounts of carbon are oxidized by the elemental selenium, forming oxidized carbon species, the electrical resistivity of carbon skeleton may increase substantially, so the electrical conducting pathway for the electron delivery and electron harvesting during each electrochemical cycle is imparted, eventually reaching a level that rechargeable battery would not functional properly, not to mention the losses of electrical energy and chemical energy into heat, which is indeed not desirable for a rechargeable battery in aspects of (1) low energy efficiency and (2) thermal management.

In contrast, the Se—C composite that is made with carbon skeleton having an adequate level of oxygen species does not show any change in thermal gravimetric analysis (TGA) behaviors between the fresh sample and the one aged under ambient conditions for about two years and a half; there is no exothermal weight loss at a temperature around 200-250° C.; the total weight loss of TGA is similar to the level of selenium loading (by weight) when it was freshly made; there is no gain in oxygen content; and there is no decrease in density. Therefore, a Se—C composite with a carbon skeleton having adequate amounts of oxygen species somehow achieves a level of satisfactory immobilization of selenium by applying strong chemical interactions between carbon and selenium with interfacial oxygen species, improving the oxidation stability of the Se—C composite.

Effective immobilization of selenium in carbon skeleton may be achieved by the presence of adequate amounts of oxygen species in Se—C composite possibly as an interfacial species which enable strong chemical interactions between carbon skeleton and selenium (in any chemical form). The amounts of desirable oxygen species in the immobilized selenium carbon composite may depend on the level of oxygen species in the carbon skeleton source and the level of selenium loading (by weight) in the Se—C composite. For a selenium loading of 50% or lower, the amount of the oxygen species in the Se—C composite is ≥0.63 mmol/g, ≥0.94 mmol/g, ≥1.56 mmol/g, or ≥2.19 mmol/g; for a selenium loading between 50% and 60% (including 60%), the amount of the oxygen species in the Se—C composite is ≥0.5 mmol/g, ≥0.75 mmol/g, ≥1.25 mmol/g, or ≥1.75 mmol/g; for a selenium loading 60% or higher, the amount of the oxygen species in the Se—C composite is ≥0.31 mmol/g, ≥0.47 mmol/g, ≥0.78 mmol/g, or ≥1.09 mmol/g.

Please note that adequate amounts of oxygen species in Se—C composite are not related to oxygen species involving in the exothermal weight loss at the temperatures between 200° C. and 250° C. The oxygen related to the exothermal weight loss at temperatures between 200° C. and 250° C. may very well result from the post-oxidation of the Se—C composite under ambient conditions by oxygen species like oxygen and/or moistures in air.

The amount of oxygen species in the porous carbon is usually generated during a porous carbon making process, typically comprising a carbonization process in which the precursor is converted into carbon, typically at a temperature lower than 700° C., more preferably lower than 650° C., further preferably less than 600° C., followed by an activation process in which the carbon is activated into porous carbon, typically at a temperature greater than 700° C., more preferably greater than 750° C., perhaps, around 800° C. or higher. Amounts of porosity of porous carbon depend on the degree of activation. The degree of activation is controlled by activation temperature and activation time with different activation chemicals such as water steam, CO₂, a base (such as NaOH, KOH, etc.), a salt (such as K₂CO₃, Na₂CO₃, ZnCl₂, etc.). The more severe the activation conditions are, the more porosity is generated for the porous carbon. Activation at a higher temperature, such as around 1,000° C., and/or for a certain period of time leads to a porous carbon having a higher level of porosity, which is desirable. However, activation at a higher temperature and/or for a longer period of time leads to a higher level of loss of oxygen species for a porous carbon, resulting in a lower level of oxygen species in the porous carbon, which is not desirable. A higher level of oxygen species may be preserved for the porous carbon by its activation at a relatively lower temperature and/or for a shorter period of time, which may result in a lower porosity, which is not desirable. The present disclosure contemplates the spirit of the invention with regards to the culmination of both the amounts of porosity and the amount of oxygen species for the porous carbon for selenium immobilization with a desirably high level loading of selenium with a desirably high level of immobilization characterized by an elevated activation energy, an elevated collision frequency, or a depressed level of kinetic rate constant.

As described in the previous paragraph, activation at a higher activation temperature and/or for a longer period of time leads to a higher level of oxygen species' loss. One of compromised approach may be activated at a relatively lower temperature, as an example, around 800° C. and lower, and for relatively longer period of time, as an example, 10 minutes or longer. The present invention also embodies post-treatments of the porous carbon with a desirably high level of porosity resulting from severe activation process, which leads to an enhancement in the amounts of oxygen species in the porous carbon. Such oxygen-species enhanced porous carbon then has desirable levels of both the amounts of porosity and the amounts of oxygen species, as described in the previous paragraphs.

Porous carbon may be post-treated with an oxidizing agent. The oxidizing agent may be an oxygen-including chemical. The post-treatment(s) of the porous carbon may be carried out in a liquid phase, or in a gas phase. Liquid phase post-treatments of porous carbon may be carried out in an aqueous environment. Liquid phase post-treatments of porous carbon may also be carried out in an organic solvent that may be hydrophobic or hydrophilic in nature. Liquid phase post-treatments of porous carbon may also be carried out under salt melt conditions.

The post-treatments of porous carbon may be carried in the liquid at a relatively low temperature, as an example, at the boiling point of the liquid medium or lower. Under an elevated pressure which is greater than an atmospheric pressure, the post-treatment temperature may be greater than the atmospheric boiling point of the liquid medium. Under vacuum, the post-treatment temperature may need to be less than the atmospheric boiling point of the liquid medium.

The oxidizing agent may be nitric acid, hydrogen peroxide, a salt of persulfate (as an example, ammonium persulfate, sodium persulfate, or potassium persulfate), a salt of manganese, vanadium, chromium, or other transition metal related element with a high oxidation valence state that allows to be reduced. The oxidizing agent may be oxygen, ozone, or an organic peroxide.

The post-treatments of the porous carbon in an aqueous environment may be carried out under an acidic condition, a pH-neutral condition, or a basic condition.

The post-treated porous carbon may be washed and dried. The dried post-treated porous carbon may be further treated at an elevated temperature (e.g., ≥150° C., ≥200° C., or ≥250° C.) under static condition, a flow of gas, more preferably, an inert gas. Such heat-treatment may rebalance the distributions of oxygen species related to CO₂ formation, oxygen species related to CO formation, and oxygen species related to H₂O formation.

The post-treatments of porous carbon may enhance the amount of oxygen species by ≥30%, ≥50%, ≥100%, or ≥150%.

It may be more desirable to carry out the immobilization of selenium at a temperature that preserves the oxygen species that may serve as a critical site that enhances the chemical interactions of selenium and carbon skeleton. The temperature of melting selenium into the pores of porous carbon may be at the melting point of selenium or higher. Doping with other element such as S, Te, or some other impurities, the melting point of selenium may be lowered.

Selenium (with or without a dopant) may be loaded onto the carbon skeleton by a method of impregnation. Selenium may be dissolved in a solvent, forming a selenium solution. The selenium solution is then impregnated on to a porous carbon, followed by solvent removal by evaporation, leaving the selenium behind inside the porous carbon. Such impregnation process may be repeated for the purpose of achieving adequate amounts of selenium loading. This method may also substantially lower the temperature of loading selenium on to porous carbon to a level lower than the melting point of selenium.

The source for producing the porous carbon for selenium immobilization may be from renewable carbon sources, for example, but not limited to, biomass, such as, sugar, glucose, starch, proteins, soybean meal, nuts, shells (from nuts, rice, wheat, etc.), fibers and sawdust from trees, or any carbon source related to nature. The source for producing the porous carbon for selenium immobilization may be a salt that includes carbon, as described in the previous sections. The source for producing the porous carbon for selenium immobilization may be an acid that includes carbon, such as citric acid, gluconic acid, tannic acid. The carbon source may also be from a chemical, for example, a polyol, or a polymer like polyacryonitrile or polyphenol.

Description of carbon making process.

The carbon material described herein can be obtained from a preparation method that comprises the following steps:

(1) Mixing the different ingredients: inert salt, activating agent and carbon precursor. The mixing process can comprise a ball-milling process or a freeze-drying process of a solution of the different ingredients; and (2) Carbonize the mixture under inert atmosphere in high temperature, then wash with hot water to remove inorganic salts, and dry to obtain a 3D porous material comprising interconnected curved thin carbon layers. In step (1), the inert salt can be selected from potassium chloride, sodium chloride or sodium carbonate. The activating agent can be selected from potassium carbonate, potassium bicarbonate or potassium oxalate. The carbon precursor can be selected from the renewable carbon sources described above. In step (2), the high temperature carbonization can be performed at 800-900° C., desirably, 800-850° C.; carbonization time for 1-8 hours, desirably for 1-4 hours.

The present invention embodies two main processes of making thin-wall interconnected porous carbon, self-templated and foreign-templated.

Self-templated process may involve using a salt that includes carbon and can be carbonized, as an example, potassium citrate, potassium gluconate, potassium tartrate, calcium citrate, sodium citrate, etc. When the salt is heated up, it first goes through melting process accompanying decomposition, forming water, carbon dioxide, carbon monoxide, light hydrocarbons, and some tar-like sticky and smelly oils that may be of oxygenated hydrocarbons; as the decomposition progresses, the cation of the original salt may be in a soluble form with anion of newly-formed carbonate, and/or, oxalate, and/or other form of salt; when the concentration of the newly formed carbonate, bicarbonate, oxalate, and/or other anions reaches a saturation point of the solubility, the conditions of oversaturation of the newly formed carbonate, bicarbonate, oxalate and/or other anion starts to build as decomposition process continues to progress; please note that oversaturation of solubility of a salt is quasi-stable thermodynamically; when reaching a point of oversaturation that is no more (or no longer) kinetically stable, it results in crystallization of the newly formed salt of carbonate, bicarbonate, oxalate, and/or other anion. A salt of organic carboxylate comprising a hydroxyl group (as an example, potassium citrate, potassium gluconate, potassium tartrate, etc.) may be an inhibitor of crystallization; such crystallization inhibitor allows building a higher level of oversaturation, which typically leads to a large population of crystallites that are small in size. The newly-crystallized salt of carbonate, bicarbonate, or in any other form, may be very uniform in particle sizes. As the decomposition continues as temperature increases and/or time progresses, the viscosity of melt may get higher, which may uniformly coat the surface of newly-formed crystallites of carbonate, bicarbonate, oxalate, and/or other form of salt. Then carbon eventually solidifies on the surfaces of the newly-formed crystallites. This carbon templating process is described in this disclosure as self-templated process, a carbonization process. The temperature for self-templating process may be below 700° C. Such self-templating process may be carried out by ramping temperature up at a constant rate, or at varied ramping rate. The self-templating process may also be carried out by holding heating temperature constant, or called as soaking, at a temperature less than 700° C. or lower.

As the temperature increases, the newly formed, self-templated, three-dimensional interconnected thin wall carbon undergoes an activation process by the newly-formed crystallites as activation chemical. As an example, activation process may essentially be based on the high-temperature redox reaction among carbon and newly-formed crystallites such as K₂CO₃ (from potassium citrate as the feedstock), K₂CO₃+C→K₂O+2CO, which takes place at about 700° C. or greater. The consumption of carbon may result in generation of porosity (particularly, microporosity) in carbon, leading to the formation of a three-dimensional interconnected thin wall porous carbon. The activation temperature may be as high as 1,100° C. The ramping rate may range less than 1° C./min to as high as 100° C./min. A flow of inert carrier gas, for example, nitrogen or argon, may be used for porous carbon making process. A reactive gas, such as CO₂ and steam, may also be used separately or jointly for and during activation.

There may be an additional or secondary reaction that may contribute to the generation of the porosity of carbon. Some of these reaction processes may comprise: i) the gasification of carbon by CO₂ that is produced during carbonization and/or activation process, ii) the reduction of metal oxides by carbon (as an example, K₂O+C →CO+2 K), iii) the intercalation of certain newly-formed metallic species (as an example, K) into carbon layers, or iv) a catalytic effect associated to the cations, which is particularly relevant in the case of potassium. It is particularly relevant the gasification of the carbon (C+CO₂₌₂ CO) as consequence of the CO₂ generated due to decomposition of unreacted K₂CO₃ (K₂CO₃ →K₂O+CO₂) that slowly occurs at a temperature greater than 800-850° C.

Foreign-templated process may involve using foreign particles, for example, an activating agent (for example, a salt of alkali metal and carbonate, such as, K₂CO₃, an alkali metal hydroxide, such as KOH), optionally along with an inert salt (for example, KCl, NaCl, etc.) to generate abundant amounts of porosity from a carbon source. The foreign particles may be mixed with the carbon source before the carbonation or after the carbonization, more preferably, prior to the carbonization step. Such mixing of foreign particles and the carbon source may be achieved by physical blending, optionally followed by mechanical milling, or by recrystallization of the carbon source along with the chemical of the foreign particle from an aqueous solution, aqueous slurry, or counterparts using an organic liquid medium. The drying process for recrystallization process may be by evaporation, air-drying, oven-drying, spray-drying, or freeze-drying.

When the mixture is heated up in the foreign-templated process, carbonization-activation of the carbon source over a salt template particle takes place, similar to what was described in previous paragraphs about a self-templated process. During the carbonization process, the carbon source may somehow melt and experience decompositions as temperature increases and time progresses, forming side products such as CO₂, CO, water, light hydrocarbons, and some sticky and gooey heavy hydrocarbons. The melt viscosity may increase and the melt may, eventually, coat and solidify on the surface of the foreign particles, forming interconnected thin-wall carbon coating on the surface of the foreign particles. As the temperature continue to rise, the activation process starts and gives rise to porosity, which may be essentially based on the high-temperature redox reaction, for example K₂CO₃+C →K₂O+2CO, which takes place at a temperature around 700° C. or higher.

As an example, when the foreign particles optionally comprises an inert salt, as another example, KCl, the melting point of an activation chemical such as K₂CO₃ is lowered from 891° C. to, as another example, about 630° C. Due to the formation of a KCl—K₂CO₃ liquid phase system at a temperature around 630° C. or so, the activation of the interconnected thin wall carbon may be notably accelerated, possibly due to the enhanced reactivity of K₂CO₃ with the carbonized products (interconnected thin wall carbon), possibly through more intimate contacts between the activation chemical (such as K₂CO₃, in this case) and the surface of interconnected thin wall carbon. This may suggest that the foreign-templated process with foreign particles comprising an activation chemical and, optionally, an inert salt may indeed be a more viable strategy to achieve the synthesis of a three-dimensional interconnected thin wall porous carbon with sufficient amounts of porosity while preserving adequate amounts of oxygen-including species (activation under less severe conditions, i.e., at a lower activation temperature). At the same time, the surface of the carbon surface may be isolated by the melted KCl—K₂CO₃ liquid phase, preventing the secondary reaction of the formed CO₂ and the formed interconnected thin wall porous carbon, by, possibly, restricting the diffusion of the formed CO₂ gaseous molecules to the surface of the solidified three-dimensional interconnected thin wall porous carbon, which is desirable for attaining a higher yield of interconnected thin wall porous carbon. In addition, the “monolithic” melted KCl—K₂CO₃ liquid phase may also restrict gaseous diffusion of the emitted gases like carbon volatiles out, which may result in a possibility of their re-deposition on three-dimensional interconnected thin wall porous carbon and an increase in the yield of interconnected thin wall porous carbon, which is desirable.

Example 14. Three-Dimensional Interconnected Thin Wall Porous Carbon Nanomaterial: Carbon Making and Characterizations—by a Foreign-Templating Process: Freeze-Drying of the Aqueous Mixture of Carbon Source Like Glucose, Activation Chemical Like K₂CO₃, And, Optionally, an Inert Salt Like KCl

After dissolving in distilled water appropriate amounts of KCl, K₂CO₃ and glucose, the solution was frozen with liquid nitrogen (−196° C.), then transferred to a lyophilizer, and freeze-dried at a temperature of −50° C. and at a pressure of 0.06 mbar, resulting in a solid mixture of KCl, K₂CO₃, and glucose. Then the solid mixture is carbonized at 850° C. for 1 hour under an inert atmosphere, and cooled down to room temperature, followed by washing with hot distilled water, filtration and drying, resulting in a three-dimensional interconnected thin wall porous carbon nanomaterial shown in FIG. 20.

The surface area of the three-dimensional interconnected thin wall porous carbon nanomaterial derived from glucose was measured to be 2,316 m²/g with a total pore volume of 1.04 cm³/g and a micropore volume of 0.90 cm³/g, having about 86% of pores described as microporosity and about 14% of pores described as meso and macroporosity. The N₂ adsorption isotherm and the pore size distributions of the three-dimensional interconnected thin wall porous carbon nanomaterial are shown in FIG. 21.

In another example, soybean meal is used instead of glucose. As can be seen in FIG. 22, the macroscopic structure of the three-dimensional interconnected thin wall porous carbon nanomaterial derived from soy bean meal is similar to the one derived from glucose. However, this material has a surface area of 2,613 m²/g, a pore volume of 1.42 cm³/g and a micropore volume of 1.0 cm³/g, having about 70% of pores described as microporosity and about 30% of pores described as meso and macroporosity. The N₂ adsorption isotherm and pore size distributions of this material are shown in FIG. 23. The amount of the mesoporosity of this material derived from soy bean meal is higher than that for the one derived from glucose.

TABLE 5 Activating/ Precursor/KCL/ Carbonization Templating K₂CO₃ weight Temperature Carbon Precursor agent ratio (° C.) SM-K-800 Soybean KCl 1/6.7/0 800 SM-CK-800 meal K₂CO₃ 1/0/1 800 SM-800 KCl + K₂CO₃ 1/6.7/1 800 SM-850 KCl + K₂CO₃ 1/6.7/1 850 G-K-800 Glucose KCl 1/6.7/1 800 G-CK-800 K₂CO₃ 1/0/1 800 G-800 KCl + K₂CO₃ 1/6.7/1 800 G-850 KCl + K₂CO₃ 1/6.7/1 850 Textural properties Carbon S_(BET) (m²g⁻¹) V_(p) (cm³g⁻¹) V_(micro) (cm³g⁻¹) SM-K-800 470 0.38 0.16 SM-CK-800 2650 1.22 1.04 SM-800 2580 1.19 0.98 SM-850 2613 1.42 1.00 G-K-800 335 0.16 0.12 G-CK-800 1443 0.58 0.53 G-800 2000 0.81 0.71 G-850 2316 1.04 0.90 Chemical composition (wt %) Carbon C O N SM-K-800 75.4 15.7 7.8 SM-CK-800 91.0 7.9 0.73 SM-800 93.6 5.6 0.46 SM-850 93.7 5.6 0.36 G-K-800 G-CK-800 G-800 92.7 6.8 0.14 G-850 93.6 5.9 0.19

The above Table 5 shows a number of different examples of Three-Dimensional Interconnected Thin Wall Porous Carbon Nanomaterial made by foreign-templating for different combinations of precursors; activating/templating agent; precursor/activating/templating agent weight ratio; and carbonization ratio—resulting in the different combinations of textural properties and chemical composition shown, wherein V_(BET)=Apparent Surface Area; V_(p)=Total Pore Volume; and V_(micro)=Micro-Pore Volume.

Example 15: Three-Dimensional Interconnected Thin Wall Porous Carbon Nanomaterial: Capacitor Making and Performances

The three-dimensional interconnected thin wall porous carbon nanomaterial materials in Example 14 may be used as electrodes in electrochemical capacitors. The electrode includes an organic binder, which can be, for example, PTFE or PVDF. As an option, the electrode also includes an electric-conductivity promoter such as, for example, carbon black, graphene, carbon nanotubes, etc. In a typical example, a mixture of 85 wt % of active material, 10 wt % of binder and 5% of electric-conductivity promoter is prepared.

The method of making the electrodes can include the preparation of a slurry of the different components or their dry mixture in a mortar. The slurry can be coated onto a current collector or can be shaped into self-supported electrodes like disk electrodes. The electrodes may then be pressed or roller-milled prior to their use in an electrochemical capacitor.

A symmetric electrochemical capacitor can be assembled using the above described electrodes. Two electrodes of the same mass and thickness can be used. The current collectors can be made of gold, stainless steel, aluminum, nickel, etc., depending on the electrolyte used.

The electrolyte can be an aqueous acid solution such as sulfuric or chloridric acid, a basic solution such as sodium hydroxide or potassium hydroxide, a salt such as lithium sulfate, sodium sulfate, potassium sulfate, etc.; the electrolyte can be an organic solution such as an organic salt, for example, tetramethylammonium tetrafluoroborate, tetraethylammoniutetrafluoroborate, tetrabuthylammonium tetrafluoroborate, etc., or an organic solution like an ionic liquid such as 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-Ethyl-3-methylimidazolium tetrafluoroborate, 1-Butyl-3-methylimidazolium tetrafluoroborate, 1-Butyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)imide, etc., or a pure ionic liquid such as mentioned above.

The electrochemical capacitor can be charged at 0.2 A/g, 0.5 A/g, 1 A/g, 5 A/g, 10 A/g, 20 A/g, 50 A/g or faster. The assembled electrochemical capacitors were tested by using a computer-controlled potentiostat. The tests consisted of cyclic voltammetry experiments (CV), electrochemical impedance spectroscopy studies (EIS) and galvanostatic charge/discharge (CD) cycling tests. The cell voltage can be up to 1 V in an acid or a basic solution, up to 1.5 V in an aqueous salt solution, and up to 3 V in an organic and ionic liquid electrolyte solution.

In an example, a solution of 1 M H₂SO₄ is used as electrolyte. The electrochemical capacitor made with glucose derived three-dimensional interconnected thin wall porous carbon nanomaterial (called as glucose-based) has a cell capacitance of 57 F/g at 0.2 A/g and the electrochemical capacitor made with soybean meal-based three-dimensional interconnected thin wall porous carbon nanomaterial (called as soybean meal based) has a capacitance of 60 F/g. At an ultra-large current density of 110 A/g, the cell capacitance is 24 and 31 F/g respectively.

In another example, a solution of 1 M 112504 is used as electrolyte. The glucose-based electrochemical capacitor has a cell capacitance of 37 F/g at 0.2 A/g and the soybean meal-based electrochemical capacitor of 39 F/g. At a current density of 20 A/g, the cell capacitance is 25 and 29 F/g respectively.

In another example, a solution of EMImTFSI in acetonitrile (1:1 wt %) is used as an electrolyte. The glucose-based electrochemical capacitor has a cell capacitance of 36 F/g at 0.2 A/g and the soybean meal-based electrochemical capacitor of 39 F/g. At a current density of 30 A/g, the cell capacitance is 28 and 30 F/g respectively.

A Ragone-like plot of the electrochemical capacitors with the different electrolytes is shown in Fig. X8. The robustness of the electrochemical capacitors was tested by long-term cycling at 5-10 A/g over more than 5,000 cycles.

Example 16 Three-Dimensional Interconnected Thin Wall Porous Carbon Nanomaterial: Carbon Making by a Foreign-Templating Process, Blending (Optionally Followed by Milling) of the Carbon Source Such as Sugar, Glucose, Soybean Meal, or Sawdust, Activation Chemical Like K₂CO₃, And, Optionally, an Inert Salt Like KCl

Appropriate amounts of a carbon source like sugar, glucose, soybean meal, sawdust (as an example, pine sawdust, cherry sawdust, or oak sawdust), an activation chemical like K₂CO₃, and, optionally, an inert salt like were weighed into a includer, followed by physical mixing that may involve mechanical milling, followed by transferring into a crucible which may be made of ceramics, steel, stainless steel, etc. Under a flow of an inert gas (which is also optional), the mixture is heated at a ramping rate less 100° C./min, optionally to a soaking temperature equal to or less than 700° C. for a period of time for carbonization, or directly to a activation temperature that is equal to or greater than 700° C., and dwell at the temperature for a period of time for activation. Throughout the ramping process, carbon source goes through carbonization process by decomposition into CO, CO₂, water, light hydrocarbons, and heavy and gooey oily hydrocarbons. During activation process, CO, CO₂, water, and carbon volatiles may continue to form.

After activation, the mixture is mixed with water, and optionally, neutralized with an acid like hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, etc. The carbon slurry is then filtered and washed with water to a level that has a low level of electrical conductivity, as an example, less than 5,000 μS/cm, less than 3,000 μS/cm, less than 1,000 μS/cm, and, optionally with a deionized or distilled water, to a conductivity less than 300 μS/cm, less than 100 μS/cm, or even less than 50 μS/cm. The filtered cakes may be dried in an oven to dry water off.

The resulted three-dimensional interconnected thin wall porous carbon nanomaterials were then characterized with oxygen content, SEM, XRD, Raman, TGA-DSC, N₂ and CO₂ BET Surface Area and Pore Size Distributions.

The following Table 5 shows elemental analysis of three-dimensional interconnected thin wall porous carbon nanomaterials such as 237-8 (self-templated carbon synthesized from potassium citrate), 237-51A (self-templated carbon from potassium citrate), SKK2 (foreign-templated carbon) vs. commercial carbons such as Elite C available from Calgon Carbon of Pittsburgh, Pa., USA; YP-50 available from Kruray Co. Ltd of Osaka, Japan; Ketjen carbon black available from Lion Specialty Chemicals Co. Ltd. Of Tokyo, Japan; Maxsorb available from Kansai Coke and Chemicals Co. Ltd. of Hyogo, Japan.

TABLE 6 ELEMENTAL ANALYSIS (wt %) Sample ID C O H N S ELITE C 97.54 1.19 0.45 1.05 0.08 YP-50F 97.96 1.46 0.60 1.08 0.02 KETJEN 98.56 0.33 0.28 0.97 0.10 MAXSORB 96.99 1.33 0.28 1.36 0.01 237-8 87.62 9.29 0.57 1.24 0.01 237-51A 87.84 9.90 0.40 1.12 0.02 SKK2 94.30 5.25 0.31 0.14 — 237-101D 12%

The above Table 6 shows two group of carbon, one group being three-dimensional interconnected thin wall porous carbon nanomaterials such as 237-8 (self-templated carbon synthesized from potassium citrate), 237-51A (foreign-templated carbon from glucose), 237-101D (self-templated carbon synthesized from potassium citrate) and SKK2 (foreign-templated carbon from sugar) that have an oxygen content greater than 1.5 wt. %, greater than 2.0 wt. %, greater than 3% wt. %, or greater than 4.0 wt. %. XRD studies show that the carbon materials in the above table are all amorphous. Raman studies show that the carbon materials in the above table are also amorphous in nature, showing amorphous carbon signature Raman scattering both at D-peak and G-peak.

The prepared interconnected thin wall porous carbon is then used to prepare selenium immobilization. Appropriate amounts of selenium and carbon are mixed together as described previously, followed by melting the selenium into the interconnected thin wall porous carbon in the presence of elevated level of oxygen species, resulted in the immobilized selenium of the present invention; please refer to the previous description for the process of making immobilized selenium in the present disclosure. The immobilized selenium is then characterized by oxygen content analysis, XRD, Raman, SEM, TGA-DSC, etc.

Table Oxygen content analysis of the immobilized selenium in three-dimensional interconnected thin wall porous carbon nanomaterials that has an greater than 1.5 wt. %, greater than 2.0 wt. %, greater than 3% wt. %, or greater than 4.0 wt. %. vs. the immobilized carbon-selenium composite that is prepared with commercial carbons that has an oxygen content less than 1.5%, less than 2.0 wt. %, less than 3% wt. %, or less than 4.0 wt. %.

TABLE 7 Carbon ID C-Se ID O (%) Elite C 115 24 Maxsorb MSP20X 259 14 Ketjen 600JD 352 0.47 237-8 301 2.6 237-51A 366 3.1 237-67 426 4.9 237-101D 483 4.8 SKK-2 385 4.8

The above Table 7 shows two different groups of immobilized selenium one group (called Group I, namely, Ketjen 600JD, 237-8, 237-51A, 237-67, 237-101D and SKK-2) having an oxygen content that is roughly a half of the oxygen content for the three-dimensional interconnected thin wall porous carbon nanomaterials, ranging from 1% to 10%, from 1.5% to 9%, or from 2% to 8%, another group (called Group II, namely, Elite C and Maxsorb MSP20X) having an oxygen content that is at least five times the oxygen content of the carbon, or greater that 8%, greater than 9%, or greater than 10%. Please note that Ketjun carbon is non-porous carbon, whose carbon-selenium composite is considered non-immobilized selenium; therefore its oxygen content stays very low, less than 1%.

The oxygen species in the Group I immobilized selenium may play critical roles as interfacial chemical groups in immobilization of selenium, allowing strong interactions between carbon and selenium, resulting in a tighter packing of selenium atoms or species inside the pores of the three-dimensional interconnected thin wall porous carbon nanomaterials, which may be evidenced by an enhanced collision frequency shown in examples of the present disclosure. Such interactions between carbon skeleton and selenium atoms or selenium chemical species, may also be further evidenced a higher intrinsic density.

As an example, a sample of the immobilized selenium that is made from three-dimensional interconnected thin wall porous carbon nanomaterials, for example, self-templated carbon synthesized from potassium citrate, at a 50-50 (by weight) ratio of carbon to selenium. It was surprised to discover its density to be 3.42 g/cm³. Please note that selenium has a density of 4.819 g/cm³. By using 4.819 g/cm³, the carbon density in the composite of the immobilized selenium is calculated to be about 2.8 g/cm³. Please also note that diamond has a density of 3.5 g/cm³; graphite has a density of about 2.3 g/cm³; and amorphous carbon has a typical density of about 2.0 g/cm³.

The immobilized selenium is then used to make a cathode of a rechargeable battery by mixing appropriate amounts of immobilized selenium, an aqueous or organic medium, and at least one binder, as described previously, followed by coating onto a charge collector, as an example, aluminum foil, resulting in a cathode comprising immobilized selenium. Please refer to the previous description for the process of making a cathode comprising immobilized selenium in the present disclosure.

The resulted cathode comprising immobilized selenium is then used to assemble a rechargeable battery with an anode, as an example, lithium, a separator, an electrolyte. Please refer to the previous description for the process of assembling a rechargeable battery comprising a cathode comprising immobilized selenium in the present disclosure.

The resulted carbon sample may be further used in an electrode making process for a capacitor or for a rechargeable battery.

The examples have been described with reference to the accompanying figures. Modifications and alterations will occur to others upon reading and understanding the foregoing examples. Accordingly, the foregoing examples are not to be construed as limiting the disclosure. 

The invention claimed is:
 1. A method of preparing an immobilized selenium body comprising: (a) forming a mixture of selenium, carbon, and oxygen; (b) heating the mixture of step (a) to a temperature above the melting temperature of selenium; and (c) causing the heated mixture of step (b) to cool to ambient or room temperature, thereby forming the immobilized selenium body.
 2. The method of claim 1, wherein the carbon is one of a self-templated carbon or a foreign-templated carbon.
 3. The method of claim 1, wherein, for selenium loading in the carbon of ≤50%, the amount of oxygen in the mixture is ≥0.63 milli-mole/gram.
 4. The method of claim 1, wherein, for selenium loading in the carbon between 50% (exclusive) and 60% (inclusive) (i.e., 50%<selenium loading ≤60%), the amount of oxygen in the mixture is ≥0.5 milli-mole/gram.
 5. The method of claim 1, wherein, for selenium loading in the carbon of ≥60%, the amount of oxygen in the mixture is ≥0.31 milli-mole/gram.
 6. The method of claim 1, further including, prior to step (a), activating the carbon thereby forming pores in the carbon.
 7. The method of claim 5, further including, combining or mixing the activated carbon with an oxidizing agent.
 8. The method of claim 7, wherein the oxidizing agent comprises at least one of the following: nitric acid; hydrogen peroxide; organic peroxide; oxygen; and/or ozone.
 9. The method of claim 7, wherein the oxidizing agent comprises at least one of the following in an aqueous environment: ammonium persulfate; sodium persulfate; potassium persulfate; a salt of manganese; a salt of vanadium, and/or a salt of chromium.
 10. The method of claim 1, wherein the self-templated carbon is made by a process that includes: carbonizing a salt by melting until oversaturation of anions in the melt forms a crystallized salt in the melt; increasing the viscosity of the melt until the melt coats surfaces of crystallites of the crystallized salt; and solidifying carbon in the melt on the surfaces of the crystallites.
 11. An immobilized selenium body comprising a mixture of: selenium; carbon; and oxygen. 